path: root/guix/gexp.scm

;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2014, 2015, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
;;;
;;; This file is part of GNU Guix.
;;;
;;; GNU Guix is free software; you can redistribute it and/or modify it
;;; under the terms of the GNU General Public License as published by
;;; the Free Software Foundation; either version 3 of the License, or (at
;;; your option) any later version.
;;;
;;; GNU Guix is distributed in the hope that it will be useful, but
;;; WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;;; GNU General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.

(define-module (guix gexp)
  #:use-module (guix store)
  #:use-module (guix monads)
  #:use-module (guix derivations)
  #:use-module (guix grafts)
  #:use-module (guix utils)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-9)
  #:use-module (srfi srfi-9 gnu)
  #:use-module (srfi srfi-26)
  #:use-module (ice-9 match)
  #:export (gexp
            gexp?
            with-imported-modules

            gexp-input
            gexp-input?

            local-file
            local-file?
            local-file-file
            local-file-absolute-file-name
            local-file-name
            local-file-recursive?

            plain-file
            plain-file?
            plain-file-name
            plain-file-content

            computed-file
            computed-file?
            computed-file-name
            computed-file-gexp
            computed-file-options

            program-file
            program-file?
            program-file-name
            program-file-gexp
            program-file-guile

            scheme-file
            scheme-file?
            scheme-file-name
            scheme-file-gexp

            file-append
            file-append?
            file-append-base
            file-append-suffix

            gexp->derivation
            gexp->file
            gexp->script
            text-file*
            mixed-text-file
            imported-files
            imported-modules
            compiled-modules

            define-gexp-compiler
            gexp-compiler?
            lower-object

            lower-inputs))

;;; Commentary:
;;;
;;; This module implements "G-expressions", or "gexps".  Gexps are like
;;; S-expressions (sexps), with two differences:
;;;
;;;   1. References (un-quotations) to derivations or packages in a gexp are
;;;      replaced by the corresponding output file name; in addition, the
;;;      'ungexp-native' unquote-like form allows code to explicitly refer to
;;;      the native code of a given package, in case of cross-compilation;
;;;
;;;   2. Gexps embed information about the derivations they refer to.
;;;
;;; Gexps make it easy to write to files Scheme code that refers to store
;;; items, or to write Scheme code to build derivations.
;;;
;;; Code:

;; "G expressions".
(define-record-type <gexp>
  (make-gexp references modules proc)
  gexp?
  (references gexp-references)                    ;list of <gexp-input>
  (modules    gexp-self-modules)                  ;list of module names
  (proc       gexp-proc))                         ;procedure

(define (write-gexp gexp port)
  "Write GEXP on PORT."
  (display "#<gexp " port)

  ;; Try to write the underlying sexp.  Now, this trick doesn't work when
  ;; doing things like (ungexp-splicing (gexp ())) because GEXP's procedure
  ;; tries to use 'append' on that, which fails with wrong-type-arg.
  (false-if-exception
   (write (apply (gexp-proc gexp)
                 (gexp-references gexp))
          port))
  (format port " ~a>"
          (number->string (object-address gexp) 16)))

(set-record-type-printer! <gexp> write-gexp)


;;;
;;; Methods.
;;;

;; Compiler for a type of objects that may be introduced in a gexp.
(define-record-type <gexp-compiler>
  (gexp-compiler type lower expand)
  gexp-compiler?
  (type       gexp-compiler-type)                 ;record type descriptor
  (lower      gexp-compiler-lower)
  (expand     gexp-compiler-expand))              ;#f | DRV -> sexp

(define %gexp-compilers
  ;; 'eq?' mapping of record type descriptor to <gexp-compiler>.
  (make-hash-table 20))

(define (default-expander thing obj output)
  "This is the default expander for \"things\" that appear in gexps.  It
returns its output file name of OBJ's OUTPUT."
  (match obj
    ((? derivation? drv)
     (derivation->output-path drv output))
    ((? string? file)
     file)))

(define (register-compiler! compiler)
  "Register COMPILER as a gexp compiler."
  (hashq-set! %gexp-compilers
              (gexp-compiler-type compiler) compiler))

(define (lookup-compiler object)
  "Search for a compiler for OBJECT.  Upon success, return the three argument
procedure to lower it; otherwise return #f."
  (and=> (hashq-ref %gexp-compilers (struct-vtable object))
         gexp-compiler-lower))

(define (lookup-expander object)
  "Search for an expander for OBJECT.  Upon success, return the three argument
procedure to expand it; otherwise return #f."
  (and=> (hashq-ref %gexp-compilers (struct-vtable object))
         gexp-compiler-expand))

(define* (lower-object obj
                       #:optional (system (%current-system))
                       #:key target)
  "Return as a value in %STORE-MONAD the derivation or store item
corresponding to OBJ for SYSTEM, cross-compiling for TARGET if TARGET is true.
OBJ must be an object that has an associated gexp compiler, such as a
<package>."
  (let ((lower (lookup-compiler obj)))
    (lower obj system target)))

(define-syntax define-gexp-compiler
  (syntax-rules (=> compiler expander)
    "Define NAME as a compiler for objects matching PREDICATE encountered in
gexps.

In the simplest form of the macro, BODY must return a derivation for PARAM, an
object that matches PREDICATE, for SYSTEM and TARGET (the latter of which is
#f except when cross-compiling.)

The more elaborate form allows you to specify an expander:

  (define-gexp-compiler something something?
    compiler => (lambda (param system target) ...)
    expander => (lambda (param drv output) ...))

The expander specifies how an object is converted to its sexp representation."
    ((_ (name (param record-type) system target) body ...)
     (define-gexp-compiler name record-type
       compiler => (lambda (param system target) body ...)
       expander => default-expander))
    ((_ name record-type
        compiler => compile
        expander => expand)
     (begin
       (define name
         (gexp-compiler record-type compile expand))
       (register-compiler! name)))))

(define-gexp-compiler (derivation-compiler (drv <derivation>) system target)
  ;; Derivations are the lowest-level representation, so this is the identity
  ;; compiler.
  (with-monad %store-monad
    (return drv)))


;;;
;;; File declarations.
;;;

;; A local file name.  FILE is the file name the user entered, which can be a
;; relative file name, and ABSOLUTE is a promise that computes its canonical
;; absolute file name.  We keep it in a promise to compute it lazily and avoid
;; repeated 'stat' calls.
(define-record-type <local-file>
  (%%local-file file absolute name recursive? select?)
  local-file?
  (file       local-file-file)                    ;string
  (absolute   %local-file-absolute-file-name)     ;promise string
  (name       local-file-name)                    ;string
  (recursive? local-file-recursive?)              ;Boolean
  (select?    local-file-select?))                ;string stat -> Boolean

(define (true file stat) #t)

(define* (%local-file file promise #:optional (name (basename file))
                      #:key recursive? (select? true))
  ;; This intermediate procedure is part of our ABI, but the underlying
  ;; %%LOCAL-FILE is not.
  (%%local-file file promise name recursive? select?))

(define (absolute-file-name file directory)
  "Return the canonical absolute file name for FILE, which lives in the
vicinity of DIRECTORY."
  (canonicalize-path
   (cond ((string-prefix? "/" file) file)
         ((not directory) file)
         ((string-prefix? "/" directory)
          (string-append directory "/" file))
         (else file))))

(define-syntax-rule (local-file file rest ...)
  "Return an object representing local file FILE to add to the store; this
object can be used in a gexp.  If FILE is a relative file name, it is looked
up relative to the source file where this form appears.  FILE will be added to
the store under NAME--by default the base name of FILE.

When RECURSIVE? is true, the contents of FILE are added recursively; if FILE
designates a flat file and RECURSIVE? is true, its contents are added, and its
permission bits are kept.

When RECURSIVE? is true, call (SELECT?  FILE STAT) for each directory entry,
where FILE is the entry's absolute file name and STAT is the result of
'lstat'; exclude entries for which SELECT? does not return true.

This is the declarative counterpart of the 'interned-file' monadic procedure."
  (%local-file file
               (delay (absolute-file-name file (current-source-directory)))
               rest ...))

(define (local-file-absolute-file-name file)
  "Return the absolute file name for FILE, a <local-file> instance.  A
'system-error' exception is raised if FILE could not be found."
  (force (%local-file-absolute-file-name file)))

(define-gexp-compiler (local-file-compiler (file <local-file>) system target)
  ;; "Compile" FILE by adding it to the store.
  (match file
    (($ <local-file> file (= force absolute) name recursive? select?)
     ;; Canonicalize FILE so that if it's a symlink, it is resolved.  Failing
     ;; to do that, when RECURSIVE? is #t, we could end up creating a dangling
     ;; symlink in the store, and when RECURSIVE? is #f 'add-to-store' would
     ;; just throw an error, both of which are inconvenient.
     (interned-file absolute name
                    #:recursive? recursive? #:select? select?))))

(define-record-type <plain-file>
  (%plain-file name content references)
  plain-file?
  (name        plain-file-name)                   ;string
  (content     plain-file-content)                ;string
  (references  plain-file-references))            ;list (currently unused)

(define (plain-file name content)
  "Return an object representing a text file called NAME with the given
CONTENT (a string) to be added to the store.

This is the declarative counterpart of 'text-file'."
  ;; XXX: For now just ignore 'references' because it's not clear how to use
  ;; them in a declarative context.
  (%plain-file name content '()))

(define-gexp-compiler (plain-file-compiler (file <plain-file>) system target)
  ;; "Compile" FILE by adding it to the store.
  (match file
    (($ <plain-file> name content references)
     (text-file name content references))))

(define-record-type <computed-file>
  (%computed-file name gexp options)
  computed-file?
  (name       computed-file-name)                 ;string
  (gexp       computed-file-gexp)                 ;gexp
  (options    computed-file-options))             ;list of arguments

(define* (computed-file name gexp
                        #:key (options '(#:local-build? #t)))
  "Return an object representing the store item NAME, a file or directory
computed by GEXP.  OPTIONS is a list of additional arguments to pass
to 'gexp->derivation'.

This is the declarative counterpart of 'gexp->derivation'."
  (%computed-file name gexp options))

(define-gexp-compiler (computed-file-compiler (file <computed-file>)
                                              system target)
  ;; Compile FILE by returning a derivation whose build expression is its
  ;; gexp.
  (match file
    (($ <computed-file> name gexp options)
     (apply gexp->derivation name gexp options))))

(define-record-type <program-file>
  (%program-file name gexp guile)
  program-file?
  (name       program-file-name)                  ;string
  (gexp       program-file-gexp)                  ;gexp
  (guile      program-file-guile))                ;package

(define* (program-file name gexp #:key (guile #f))
  "Return an object representing the executable store item NAME that runs
GEXP.  GUILE is the Guile package used to execute that script.

This is the declarative counterpart of 'gexp->script'."
  (%program-file name gexp guile))

(define-gexp-compiler (program-file-compiler (file <program-file>)
                                             system target)
  ;; Compile FILE by returning a derivation that builds the script.
  (match file
    (($ <program-file> name gexp guile)
     (gexp->script name gexp
                   #:guile (or guile (default-guile))))))

(define-record-type <scheme-file>
  (%scheme-file name gexp)
  scheme-file?
  (name       scheme-file-name)                  ;string
  (gexp       scheme-file-gexp))                 ;gexp

(define* (scheme-file name gexp)
  "Return an object representing the Scheme file NAME that contains GEXP.

This is the declarative counterpart of 'gexp->file'."
  (%scheme-file name gexp))

(define-gexp-compiler (scheme-file-compiler (file <scheme-file>)
                                            system target)
  ;; Compile FILE by returning a derivation that builds the file.
  (match file
    (($ <scheme-file> name gexp)
     (gexp->file name gexp))))

;; Appending SUFFIX to BASE's output file name.
(define-record-type <file-append>
  (%file-append base suffix)
  file-append?
  (base   file-append-base)                    ;<package> | <derivation> | ...
  (suffix file-append-suffix))                 ;list of strings

(define (file-append base . suffix)
  "Return a <file-append> object that expands to the concatenation of BASE and
SUFFIX."
  (%file-append base suffix))

(define-gexp-compiler file-append-compiler <file-append>
  compiler => (lambda (obj system target)
                (match obj
                  (($ <file-append> base _)
                   (lower-object base system #:target target))))
  expander => (lambda (obj lowered output)
                (match obj
                  (($ <file-append> base suffix)
                   (let* ((expand (lookup-expander base))
                          (base   (expand base lowered output)))
                     (string-append base (string-concatenate suffix)))))))


;;;
;;; Inputs & outputs.
;;;

;; The input of a gexp.
(define-record-type <gexp-input>
  (%gexp-input thing output native?)
  gexp-input?
  (thing     gexp-input-thing)       ;<package> | <origin> | <derivation> | ...
  (output    gexp-input-output)      ;string
  (native?   gexp-input-native?))    ;Boolean

(define (write-gexp-input input port)
  (match input
    (($ <gexp-input> thing output #f)
     (format port "#<gexp-input ~s:~a>" thing output))
    (($ <gexp-input> thing output #t)
     (format port "#<gexp-input native ~s:~a>" thing output))))

(set-record-type-printer! <gexp-input> write-gexp-input)

(define* (gexp-input thing                        ;convenience procedure
                     #:optional (output "out")
                     #:key native?)
  "Return a new <gexp-input> for the OUTPUT of THING; NATIVE? determines
whether this should be considered a \"native\" input or not."
  (%gexp-input thing output native?))

;; Reference to one of the derivation's outputs, for gexps used in
;; derivations.
(define-record-type <gexp-output>
  (gexp-output name)
  gexp-output?
  (name gexp-output-name))

(define (write-gexp-output output port)
  (match output
    (($ <gexp-output> name)
     (format port "#<gexp-output ~a>" name))))

(set-record-type-printer! <gexp-output> write-gexp-output)

(define (gexp-modules gexp)
  "Return the list of Guile module names GEXP relies on."
  (delete-duplicates
   (append (gexp-self-modules gexp)
           (append-map (match-lambda
                         (($ <gexp-input> (? gexp? exp))
                          (gexp-modules exp))
                         (($ <gexp-input> (lst ...))
                          (append-map (lambda (item)
                                        (if (gexp? item)
                                            (gexp-modules item)
                                            '()))
                                      lst))
                         (_
                          '()))
                       (gexp-references gexp)))))

(define* (lower-inputs inputs
                       #:key system target)
  "Turn any package from INPUTS into a derivation for SYSTEM; return the
corresponding input list as a monadic value.  When TARGET is true, use it as
the cross-compilation target triplet."
  (with-monad %store-monad
    (sequence %store-monad
              (map (match-lambda
                     (((? struct? thing) sub-drv ...)
                      (mlet %store-monad ((drv (lower-object
                                                thing system #:target target)))
                        (return `(,drv ,@sub-drv))))
                     (input
                      (return input)))
                   inputs))))

(define* (lower-reference-graphs graphs #:key system target)
  "Given GRAPHS, a list of (FILE-NAME INPUT ...) lists for use as a
#:reference-graphs argument, lower it such that each INPUT is replaced by the
corresponding derivation."
  (match graphs
    (((file-names . inputs) ...)
     (mlet %store-monad ((inputs (lower-inputs inputs
                                               #:system system
                                               #:target target)))
       (return (map cons file-names inputs))))))

(define* (lower-references lst #:key system target)
  "Based on LST, a list of output names and packages, return a list of output
names and file names suitable for the #:allowed-references argument to
'derivation'."
  (with-monad %store-monad
    (define lower
      (match-lambda
       ((? string? output)
        (return output))
       (($ <gexp-input> thing output native?)
        (mlet %store-monad ((drv (lower-object thing system
                                               #:target (if native?
                                                            #f target))))
          (return (derivation->output-path drv output))))
       (thing
        (mlet %store-monad ((drv (lower-object thing system
                                               #:target target)))
          (return (derivation->output-path drv))))))

    (sequence %store-monad (map lower lst))))

(define default-guile-derivation
  ;; Here we break the abstraction by talking to the higher-level layer.
  ;; Thus, do the resolution lazily to hide the circular dependency.
  (let ((proc (delay
                (let ((iface (resolve-interface '(guix packages))))
                  (module-ref iface 'default-guile-derivation)))))
    (lambda (system)
      ((force proc) system))))

(define* (gexp->derivation name exp
                           #:key
                           system (target 'current)
                           hash hash-algo recursive?
                           (env-vars '())
                           (modules '())
                           (module-path %load-path)
                           (guile-for-build (%guile-for-build))
                           (graft? (%graft?))
                           references-graphs
                           allowed-references disallowed-references
                           leaked-env-vars
                           local-build? (substitutable? #t)
                           (script-name (string-append name "-builder")))
  "Return a derivation NAME that runs EXP (a gexp) with GUILE-FOR-BUILD (a
derivation) on SYSTEM; EXP is stored in a file called SCRIPT-NAME.  When
TARGET is true, it is used as the cross-compilation target triplet for
packages referred to by EXP.

MODULES is deprecated in favor of 'with-imported-modules'.  Its meaning is to
make MODULES available in the evaluation context of EXP; MODULES is a list of
names of Guile modules searched in MODULE-PATH to be copied in the store,
compiled, and made available in the load path during the execution of
EXP---e.g., '((guix build utils) (guix build gnu-build-system)).

GRAFT? determines whether packages referred to by EXP should be grafted when
applicable.

When REFERENCES-GRAPHS is true, it must be a list of tuples of one of the
following forms:

  (FILE-NAME PACKAGE)
  (FILE-NAME PACKAGE OUTPUT)
  (FILE-NAME DERIVATION)
  (FILE-NAME DERIVATION OUTPUT)
  (FILE-NAME STORE-ITEM)

The right-hand-side of each element of REFERENCES-GRAPHS is automatically made
an input of the build process of EXP.  In the build environment, each
FILE-NAME contains the reference graph of the corresponding item, in a simple
text format.

ALLOWED-REFERENCES must be either #f or a list of output names and packages.
In the latter case, the list denotes store items that the result is allowed to
refer to.  Any reference to another store item will lead to a build error.
Similarly for DISALLOWED-REFERENCES, which can list items that must not be
referenced by the outputs.

The other arguments are as for 'derivation'."
  (define %modules
    (delete-duplicates
     (append modules (gexp-modules exp))))
  (define outputs (gexp-outputs exp))

  (define (graphs-file-names graphs)
    ;; Return a list of (FILE-NAME . STORE-PATH) pairs made from GRAPHS.
    (map (match-lambda
          ;; TODO: Remove 'derivation?' special cases.
           ((file-name (? derivation? drv))
            (cons file-name (derivation->output-path drv)))
           ((file-name (? derivation? drv) sub-drv)
            (cons file-name (derivation->output-path drv sub-drv)))
           ((file-name thing)
            (cons file-name thing)))
         graphs))

  (mlet* %store-monad (;; The following binding forces '%current-system' and
                       ;; '%current-target-system' to be looked up at >>=
                       ;; time.
                       (graft?    (set-grafting graft?))

                       (system -> (or system (%current-system)))
                       (target -> (if (eq? target 'current)
                                      (%current-target-system)
                                      target))
                       (normals  (lower-inputs (gexp-inputs exp)
                                               #:system system
                                               #:target target))
                       (natives  (lower-inputs (gexp-native-inputs exp)
                                               #:system system
                                               #:target #f))
                       (inputs -> (append normals natives))
                       (sexp     (gexp->sexp exp
                                             #:system system
                                             #:target target))
                       (builder  (text-file script-name
                                            (object->string sexp)))
                       (modules  (if (pair? %modules)
                                     (imported-modules %modules
                                                       #:system system
                                                       #:module-path module-path
                                                       #:guile guile-for-build)
                                     (return #f)))
                       (compiled (if (pair? %modules)
                                     (compiled-modules %modules
                                                       #:system system
                                                       #:module-path module-path
                                                       #:guile guile-for-build)
                                     (return #f)))
                       (graphs   (if references-graphs
                                     (lower-reference-graphs references-graphs
                                                             #:system system
                                                             #:target target)
                                     (return #f)))
                       (allowed  (if allowed-references
                                     (lower-references allowed-references
                                                       #:system system
                                                       #:target target)
                                     (return #f)))
                       (disallowed (if disallowed-references
                                       (lower-references disallowed-references
                                                         #:system system
                                                         #:target target)
                                       (return #f)))
                       (guile    (if guile-for-build
                                     (return guile-for-build)
                                     (default-guile-derivation system))))
    (mbegin %store-monad
      (set-grafting graft?)                       ;restore the initial setting
      (raw-derivation name
                      (string-append (derivation->output-path guile)
                                     "/bin/guile")
                      `("--no-auto-compile"
                        ,@(if (pair? %modules)
                              `("-L" ,(derivation->output-path modules)
                                "-C" ,(derivation->output-path compiled))
                              '())
                        ,builder)
                      #:outputs outputs
                      #:env-vars env-vars
                      #:system system
                      #:inputs `((,guile)
                                 (,builder)
                                 ,@(if modules
                                       `((,modules) (,compiled) ,@inputs)
                                       inputs)
                                 ,@(match graphs
                                     (((_ . inputs) ...) inputs)
                                     (_ '())))
                      #:hash hash #:hash-algo hash-algo #:recursive? recursive?
                      #:references-graphs (and=> graphs graphs-file-names)
                      #:allowed-references allowed
                      #:disallowed-references disallowed
                      #:leaked-env-vars leaked-env-vars
                      #:local-build? local-build?
                      #:substitutable? substitutable?))))

(define* (gexp-inputs exp #:key native?)
  "Return the input list for EXP.  When NATIVE? is true, return only native
references; otherwise, return only non-native references."
  (define (add-reference-inputs ref result)
    (match ref
      (($ <gexp-input> (? gexp? exp) _ #t)
       (if native?
           (append (gexp-inputs exp)
                   (gexp-inputs exp #:native? #t)
                   result)
           result))
      (($ <gexp-input> (? gexp? exp) _ #f)
       (append (gexp-inputs exp #:native? native?)
               result))
      (($ <gexp-input> (? string? str))
       (if (direct-store-path? str)
           (cons `(,str) result)
           result))
      (($ <gexp-input> (? struct? thing) output n?)
       (if (and (eqv? n? native?) (lookup-compiler thing))
           ;; THING is a derivation, or a package, or an origin, etc.
           (cons `(,thing ,output) result)
           result))
      (($ <gexp-input> (lst ...) output n?)
       (if (eqv? native? n?)
           (fold-right add-reference-inputs result
                       ;; XXX: For now, automatically convert LST to a list of
                       ;; gexp-inputs.
                       (map (match-lambda
                              ((? gexp-input? x) x)
                              (x (%gexp-input x "out" (or n? native?))))
                            lst))
           result))
      (_
       ;; Ignore references to other kinds of objects.
       result)))

  (fold-right add-reference-inputs
              '()
              (gexp-references exp)))

(define gexp-native-inputs
  (cut gexp-inputs <> #:native? #t))

(define (gexp-outputs exp)
  "Return the outputs referred to by EXP as a list of strings."
  (define (add-reference-output ref result)
    (match ref
      (($ <gexp-output> name)
       (cons name result))
      (($ <gexp-input> (? gexp? exp))
       (append (gexp-outputs exp) result))
      (($ <gexp-input> (lst ...) output native?)
       ;; XXX: Automatically convert LST.
       (add-reference-output (map (match-lambda
                                   ((? gexp-input? x) x)
                                   (x (%gexp-input x "out" native?)))
                                  lst)
                             result))
      ((lst ...)
       (fold-right add-reference-output result lst))
      (_
       result)))

  (delete-duplicates
   (add-reference-output (gexp-references exp) '())))

(define* (gexp->sexp exp #:key
                     (system (%current-system))
                     (target (%current-target-system)))
  "Return (monadically) the sexp corresponding to EXP for the given OUTPUT,
and in the current monad setting (system type, etc.)"
  (define* (reference->sexp ref #:optional native?)
    (with-monad %store-monad
      (match ref
        (($ <gexp-output> output)
         ;; Output file names are not known in advance but the daemon defines
         ;; an environment variable for each of them at build time, so use
         ;; that trick.
         (return `((@ (guile) getenv) ,output)))
        (($ <gexp-input> (? gexp? exp) output n?)
         (gexp->sexp exp
                     #:system system
                     #:target (if (or n? native?) #f target)))
        (($ <gexp-input> (refs ...) output n?)
         (sequence %store-monad
                   (map (lambda (ref)
                          ;; XXX: Automatically convert REF to an gexp-input.
                          (reference->sexp
                           (if (gexp-input? ref)
                               ref
                               (%gexp-input ref "out" n?))
                           (or n? native?)))
                        refs)))
        (($ <gexp-input> (? struct? thing) output n?)
         (let ((target (if (or n? native?) #f target))
               (expand (lookup-expander thing)))
           (mlet %store-monad ((obj (lower-object thing system
                                                  #:target target)))
             ;; OBJ must be either a derivation or a store file name.
             (return (expand thing obj output)))))
        (($ <gexp-input> x)
         (return x))
        (x
         (return x)))))

  (mlet %store-monad
      ((args (sequence %store-monad
                       (map reference->sexp (gexp-references exp)))))
    (return (apply (gexp-proc exp) args))))

(define (syntax-location-string s)
  "Return a string representing the source code location of S."
  (let ((props (syntax-source s)))
    (if props
        (let ((file   (assoc-ref props 'filename))
              (line   (and=> (assoc-ref props 'line) 1+))
              (column (assoc-ref props 'column)))
          (if file
              (simple-format #f "~a:~a:~a"
                             file line column)
              (simple-format #f "~a:~a" line column)))
        "<unknown location>")))

(define-syntax-parameter current-imported-modules
  ;; Current list of imported modules.
  (identifier-syntax '()))

(define-syntax-rule (with-imported-modules modules body ...)
  "Mark the gexps defined in BODY... as requiring MODULES in their execution
environment."
  (syntax-parameterize ((current-imported-modules
                         (identifier-syntax modules)))
    body ...))

(define-syntax gexp
  (lambda (s)
    (define (collect-escapes exp)
      ;; Return all the 'ungexp' present in EXP.
      (let loop ((exp    exp)
                 (result '()))
        (syntax-case exp (ungexp
                          ungexp-splicing
                          ungexp-native
                          ungexp-native-splicing)
          ((ungexp _)
           (cons exp result))
          ((ungexp _ _)
           (cons exp result))
          ((ungexp-splicing _ ...)
           (cons exp result))
          ((ungexp-native _ ...)
           (cons exp result))
          ((ungexp-native-splicing _ ...)
           (cons exp result))
          ((exp0 . exp)
           (let ((result (loop #'exp0 result)))
             (loop  #'exp result)))
          (_
           result))))

    (define (escape->ref exp)
      ;; Turn 'ungexp' form EXP into a "reference".
      (syntax-case exp (ungexp ungexp-splicing
                        ungexp-native ungexp-native-splicing
                        output)
        ((ungexp output)
         #'(gexp-output "out"))
        ((ungexp output name)
         #'(gexp-output name))
        ((ungexp thing)
         #'(%gexp-input thing "out" #f))
        ((ungexp drv-or-pkg out)
         #'(%gexp-input drv-or-pkg out #f))
        ((ungexp-splicing lst)
         #'(%gexp-input lst "out" #f))
        ((ungexp-native thing)
         #'(%gexp-input thing "out" #t))
        ((ungexp-native drv-or-pkg out)
         #'(%gexp-input drv-or-pkg out #t))
        ((ungexp-native-splicing lst)
         #'(%gexp-input lst "out" #t))))

    (define (substitute-ungexp exp substs)
      ;; Given EXP, an 'ungexp' or 'ungexp-native' form, substitute it with
      ;; the corresponding form in SUBSTS.
      (match (assoc exp substs)
        ((_ id)
         id)
        (_                                        ;internal error
         (with-syntax ((exp exp))
           #'(syntax-error "error: no 'ungexp' substitution" exp)))))

    (define (substitute-ungexp-splicing exp substs)
      (syntax-case exp ()
        ((exp rest ...)
         (match (assoc #'exp substs)
           ((_ id)
            (with-syntax ((id id))
              #`(append id
                        #,(substitute-references #'(rest ...) substs))))
           (_
            #'(syntax-error "error: no 'ungexp-splicing' substitution"
                            exp))))))

    (define (substitute-references exp substs)
      ;; Return a variant of EXP where all the cars of SUBSTS have been
      ;; replaced by the corresponding cdr.
      (syntax-case exp (ungexp ungexp-native
                        ungexp-splicing ungexp-native-splicing)
        ((ungexp _ ...)
         (substitute-ungexp exp substs))
        ((ungexp-native _ ...)
         (substitute-ungexp exp substs))
        (((ungexp-splicing _ ...) rest ...)
         (substitute-ungexp-splicing exp substs))
        (((ungexp-native-splicing _ ...) rest ...)
         (substitute-ungexp-splicing exp substs))
        ((exp0 . exp)
         #`(cons #,(substitute-references #'exp0 substs)
                 #,(substitute-references #'exp substs)))
        (x #''x)))

    (syntax-case s (ungexp output)
      ((_ exp)
       (let* ((escapes (delete-duplicates (collect-escapes #'exp)))
              (formals (generate-temporaries escapes))
              (sexp    (substitute-references #'exp (zip escapes formals)))
              (refs    (map escape->ref escapes)))
         #`(make-gexp (list #,@refs)
                      current-imported-modules
                      (lambda #,formals
                        #,sexp)))))))


;;;
;;; Module handling.
;;;

(define %utils-module
  ;; This file provides 'mkdir-p', needed to implement 'imported-files' and
  ;; other primitives below.  Note: We give the file name relative to this
  ;; file you are currently reading; 'search-path' could return a file name
  ;; relative to the current working directory.
  (local-file "build/utils.scm"
              "build-utils.scm"))

(define* (imported-files files
                         #:key (name "file-import")
                         (system (%current-system))
                         (guile (%guile-for-build)))
  "Return a derivation that imports FILES into STORE.  FILES must be a list
of (FINAL-PATH . FILE-NAME) pairs; each FILE-NAME is read from the file
system, imported, and appears under FINAL-PATH in the resulting store path."
  (define file-pair
    (match-lambda
     ((final-path . file-name)
      (mlet %store-monad ((file (interned-file file-name
                                               (basename final-path))))
        (return (list final-path file))))))

  (mlet %store-monad ((files (sequence %store-monad
                                       (map file-pair files))))
    (define build
      (gexp
       (begin
         (primitive-load (ungexp %utils-module))  ;for 'mkdir-p'
         (use-modules (ice-9 match))

         (mkdir (ungexp output)) (chdir (ungexp output))
         (for-each (match-lambda
                    ((final-path store-path)
                     (mkdir-p (dirname final-path))
                     (symlink store-path final-path)))
                   '(ungexp files)))))

    ;; TODO: Pass FILES as an environment variable so that BUILD remains
    ;; exactly the same regardless of FILES: less disk space, and fewer
    ;; 'add-to-store' RPCs.
    (gexp->derivation name build
                      #:system system
                      #:guile-for-build guile
                      #:local-build? #t)))

(define* (imported-modules modules
                           #:key (name "module-import")
                           (system (%current-system))
                           (guile (%guile-for-build))
                           (module-path %load-path))
  "Return a derivation that contains the source files of MODULES, a list of
module names such as `(ice-9 q)'.  All of MODULES must be in the MODULE-PATH
search path."
  ;; TODO: Determine the closure of MODULES, build the `.go' files,
  ;; canonicalize the source files through read/write, etc.
  (let ((files (map (lambda (m)
                      (let ((f (module->source-file-name m)))
                        (cons f (search-path* module-path f))))
                    modules)))
    (imported-files files #:name name #:system system
                    #:guile guile)))

(define* (compiled-modules modules
                           #:key (name "module-import-compiled")
                           (system (%current-system))
                           (guile (%guile-for-build))
                           (module-path %load-path))
  "Return a derivation that builds a tree containing the `.go' files
corresponding to MODULES.  All the MODULES are built in a context where
they can refer to each other."
  (mlet %store-monad ((modules (imported-modules modules
                                                 #:system system
                                                 #:guile guile
                                                 #:module-path
                                                 module-path)))
    (define build
      (gexp
       (begin
         (primitive-load (ungexp %utils-module))  ;for 'mkdir-p'

         (use-modules (ice-9 ftw)
                      (srfi srfi-26)
                      (system base compile))

         (define (regular? file)
           (not (member file '("." ".."))))

         (define (process-directory directory output)
           (let ((entries (map (cut string-append directory "/" <>)
                               (scandir directory regular?))))
             (for-each (lambda (entry)
                         (if (file-is-directory? entry)
                             (let ((output (string-append output "/"
                                                          (basename entry))))
                               (mkdir-p output)
                               (process-directory entry output))
                             (let* ((base   (string-drop-right
                                             (basename entry)
                                             4)) ;.scm
                                    (output (string-append output "/" base
                                                           ".go")))
                               (compile-file entry
                                             #:output-file output
                                             #:opts
                                             %auto-compilation-options))))
                       entries)))

         (set! %load-path (cons (ungexp modules) %load-path))
         (mkdir (ungexp output))
         (chdir (ungexp modules))
         (process-directory "." (ungexp output)))))

    ;; TODO: Pass MODULES as an environment variable.
    (gexp->derivation name build
                      #:system system
                      #:guile-for-build guile
                      #:local-build? #t)))


;;;
;;; Convenience procedures.
;;;

(define (default-guile)
  ;; Lazily resolve 'guile-final'.  This module must not refer to (gnu …)
  ;; modules directly, to avoid circular dependencies, hence this hack.
  (module-ref (resolve-interface '(gnu packages commencement))
              'guile-final))

(define (load-path-expression modules)
  "Return as a monadic value a gexp that sets '%load-path' and
'%load-compiled-path' to point to MODULES, a list of module names."
  (mlet %store-monad ((modules  (imported-modules modules))
                      (compiled (compiled-modules modules)))
    (return (gexp (eval-when (expand load eval)
                    (set! %load-path
                      (cons (ungexp modules) %load-path))
                    (set! %load-compiled-path
                      (cons (ungexp compiled)
                            %load-compiled-path)))))))

(define* (gexp->script name exp
                       #:key (guile (default-guile)))
  "Return an executable script NAME that runs EXP using GUILE, with EXP's
imported modules in its search path."
  (mlet %store-monad ((set-load-path
                       (load-path-expression (gexp-modules exp))))
    (gexp->derivation name
                      (gexp
                       (call-with-output-file (ungexp output)
                         (lambda (port)
                           ;; Note: that makes a long shebang.  When the store
                           ;; is /gnu/store, that fits within the 128-byte
                           ;; limit imposed by Linux, but that may go beyond
                           ;; when running tests.
                           (format port
                                   "#!~a/bin/guile --no-auto-compile~%!#~%"
                                   (ungexp guile))

                           (write '(ungexp set-load-path) port)
                           (write '(ungexp exp) port)
                           (chmod port #o555)))))))

(define* (gexp->file name exp #:key (set-load-path? #t))
  "Return a derivation that builds a file NAME containing EXP.  When
SET-LOAD-PATH? is true, emit code in the resulting file to set '%load-path'
and '%load-compiled-path' to honor EXP's imported modules."
  (match (if set-load-path? (gexp-modules exp) '())
    (()                                           ;zero modules
     (gexp->derivation name
                       (gexp
                        (call-with-output-file (ungexp output)
                          (lambda (port)
                            (write '(ungexp exp) port))))
                       #:local-build? #t
                       #:substitutable? #f))
    ((modules ...)
     (mlet %store-monad ((set-load-path (load-path-expression modules)))
       (gexp->derivation name
                         (gexp
                          (call-with-output-file (ungexp output)
                            (lambda (port)
                              (write '(ungexp set-load-path) port)
                              (write '(ungexp exp) port))))
                         #:local-build? #t
                         #:substitutable? #f)))))

(define* (text-file* name #:rest text)
  "Return as a monadic value a derivation that builds a text file containing
all of TEXT.  TEXT may list, in addition to strings, objects of any type that
can be used in a gexp: packages, derivations, local file objects, etc.  The
resulting store file holds references to all these."
  (define builder
    (gexp (call-with-output-file (ungexp output "out")
            (lambda (port)
              (display (string-append (ungexp-splicing text)) port)))))

  (gexp->derivation name builder
                    #:local-build? #t
                    #:substitutable? #f))

(define* (mixed-text-file name #:rest text)
  "Return an object representing store file NAME containing TEXT.  TEXT is a
sequence of strings and file-like objects, as in:

  (mixed-text-file \"profile\"
                   \"export PATH=\" coreutils \"/bin:\" grep \"/bin\")

This is the declarative counterpart of 'text-file*'."
  (define build
    (gexp (call-with-output-file (ungexp output "out")
            (lambda (port)
              (display (string-append (ungexp-splicing text)) port)))))

  (computed-file name build))


;;;
;;; Syntactic sugar.
;;;

(eval-when (expand load eval)
  (define* (read-ungexp chr port #:optional native?)
    "Read an 'ungexp' or 'ungexp-splicing' form from PORT.  When NATIVE? is
true, use 'ungexp-native' and 'ungexp-native-splicing' instead."
    (define unquote-symbol
      (match (peek-char port)
        (#\@
         (read-char port)
         (if native?
             'ungexp-native-splicing
             'ungexp-splicing))
        (_
         (if native?
             'ungexp-native
             'ungexp))))

    (match (read port)
      ((? symbol? symbol)
       (let ((str (symbol->string symbol)))
         (match (string-index-right str #\:)
           (#f
            `(,unquote-symbol ,symbol))
           (colon
            (let ((name   (string->symbol (substring str 0 colon)))
                  (output (substring str (+ colon 1))))
              `(,unquote-symbol ,name ,output))))))
      (x
       `(,unquote-symbol ,x))))

  (define (read-gexp chr port)
    "Read a 'gexp' form from PORT."
    `(gexp ,(read port)))

  ;; Extend the reader
  (read-hash-extend #\~ read-gexp)
  (read-hash-extend #\$ read-ungexp)
  (read-hash-extend #\+ (cut read-ungexp <> <> #t)))

;;; gexp.scm ends here