guix.texi

contains information such as the store paths referred to by each store
path, and the list of @emph{valid} store items---results of successful
builds.  This database resides in @file{@var{localstatedir}/guix/db},
where @var{localstatedir} is the state directory specified @i{via}
@option{--localstatedir} at configure time, usually @file{/var}.

The store is @emph{always} accessed by the daemon on behalf of its clients
(@pxref{Invoking guix-daemon}).  To manipulate the store, clients
connect to the daemon over a Unix-domain socket, send requests to it,
and read the result---these are remote procedure calls, or RPCs.

@quotation Note
Users must @emph{never} modify files under @file{/gnu/store} directly.
This would lead to inconsistencies and break the immutability
assumptions of Guix's functional model (@pxref{Introduction}).

@xref{Invoking guix gc, @command{guix gc --verify}}, for information on
how to check the integrity of the store and attempt recovery from
accidental modifications.
@end quotation

The @code{(guix store)} module provides procedures to connect to the
daemon, and to perform RPCs.  These are described below.  By default,
@code{open-connection}, and thus all the @command{guix} commands,
connect to the local daemon or to the URI specified by the
@code{GUIX_DAEMON_SOCKET} environment variable.

@defvr {Environment Variable} GUIX_DAEMON_SOCKET
When set, the value of this variable should be a file name or a URI
designating the daemon endpoint.  When it is a file name, it denotes a
Unix-domain socket to connect to.  In addition to file names, the
supported URI schemes are:

@table @code
@item file
@itemx unix
These are for Unix-domain sockets.
@code{file:///var/guix/daemon-socket/socket} is equivalent to
@file{/var/guix/daemon-socket/socket}.

@item guix
@cindex daemon, remote access
@cindex remote access to the daemon
@cindex daemon, cluster setup
@cindex clusters, daemon setup
These URIs denote connections over TCP/IP, without encryption nor
authentication of the remote host.  The URI must specify the host name
and optionally a port number (by default port 44146 is used):

@example
guix://master.guix.example.org:1234
@end example

This setup is suitable on local networks, such as clusters, where only
trusted nodes may connect to the build daemon at
@code{master.guix.example.org}.

The @code{--listen} option of @command{guix-daemon} can be used to
instruct it to listen for TCP connections (@pxref{Invoking guix-daemon,
@code{--listen}}).

@item ssh
@cindex SSH access to build daemons
These URIs allow you to connect to a remote daemon over
SSH@footnote{This feature requires Guile-SSH (@pxref{Requirements}).}.
A typical URL might look like this:

@example
ssh://charlie@@guix.example.org:22
@end example

As for @command{guix copy}, the usual OpenSSH client configuration files
are honored (@pxref{Invoking guix copy}).
@end table

Additional URI schemes may be supported in the future.

@c XXX: Remove this note when the protocol incurs fewer round trips
@c and when (guix derivations) no longer relies on file system access.
@quotation Note
The ability to connect to remote build daemons is considered
experimental as of @value{VERSION}.  Please get in touch with us to
share any problems or suggestions you may have (@pxref{Contributing}).
@end quotation
@end defvr

@deffn {Scheme Procedure} open-connection [@var{uri}] [#:reserve-space? #t]
Connect to the daemon over the Unix-domain socket at @var{uri} (a string).  When
@var{reserve-space?} is true, instruct it to reserve a little bit of
extra space on the file system so that the garbage collector can still
operate should the disk become full.  Return a server object.

@var{file} defaults to @var{%default-socket-path}, which is the normal
location given the options that were passed to @command{configure}.
@end deffn

@deffn {Scheme Procedure} close-connection @var{server}
Close the connection to @var{server}.
@end deffn

@defvr {Scheme Variable} current-build-output-port
This variable is bound to a SRFI-39 parameter, which refers to the port
where build and error logs sent by the daemon should be written.
@end defvr

Procedures that make RPCs all take a server object as their first
argument.

@deffn {Scheme Procedure} valid-path? @var{server} @var{path}
@cindex invalid store items
Return @code{#t} when @var{path} designates a valid store item and
@code{#f} otherwise (an invalid item may exist on disk but still be
invalid, for instance because it is the result of an aborted or failed
build.)

A @code{&nix-protocol-error} condition is raised if @var{path} is not
prefixed by the store directory (@file{/gnu/store}).
@end deffn

@deffn {Scheme Procedure} add-text-to-store @var{server} @var{name} @var{text} [@var{references}]
Add @var{text} under file @var{name} in the store, and return its store
path.  @var{references} is the list of store paths referred to by the
resulting store path.
@end deffn

@deffn {Scheme Procedure} build-derivations @var{server} @var{derivations}
Build @var{derivations} (a list of @code{<derivation>} objects or
derivation paths), and return when the worker is done building them.
Return @code{#t} on success.
@end deffn

Note that the @code{(guix monads)} module provides a monad as well as
monadic versions of the above procedures, with the goal of making it
more convenient to work with code that accesses the store (@pxref{The
Store Monad}).

@c FIXME
@i{This section is currently incomplete.}

@node Derivations
@section Derivations

@cindex derivations
Low-level build actions and the environment in which they are performed
are represented by @dfn{derivations}.  A derivation contains the
following pieces of information:

@itemize
@item
The outputs of the derivation---derivations produce at least one file or
directory in the store, but may produce more.

@item
The inputs of the derivations, which may be other derivations or plain
files in the store (patches, build scripts, etc.)

@item
The system type targeted by the derivation---e.g., @code{x86_64-linux}.

@item
The file name of a build script in the store, along with the arguments
to be passed.

@item
A list of environment variables to be defined.

@end itemize

@cindex derivation path
Derivations allow clients of the daemon to communicate build actions to
the store.  They exist in two forms: as an in-memory representation,
both on the client- and daemon-side, and as files in the store whose
name end in @code{.drv}---these files are referred to as @dfn{derivation
paths}.  Derivations paths can be passed to the @code{build-derivations}
procedure to perform the build actions they prescribe (@pxref{The
Store}).

The @code{(guix derivations)} module provides a representation of
derivations as Scheme objects, along with procedures to create and
otherwise manipulate derivations.  The lowest-level primitive to create
a derivation is the @code{derivation} procedure:

@deffn {Scheme Procedure} derivation @var{store} @var{name} @var{builder} @
  @var{args} [#:outputs '("out")] [#:hash #f] [#:hash-algo #f] @
  [#:recursive? #f] [#:inputs '()] [#:env-vars '()] @
  [#:system (%current-system)] [#:references-graphs #f] @
  [#:allowed-references #f] [#:disallowed-references #f] @
  [#:leaked-env-vars #f] [#:local-build? #f] @
  [#:substitutable? #t]
Build a derivation with the given arguments, and return the resulting
@code{<derivation>} object.

When @var{hash} and @var{hash-algo} are given, a
@dfn{fixed-output derivation} is created---i.e., one whose result is
known in advance, such as a file download.  If, in addition,
@var{recursive?} is true, then that fixed output may be an executable
file or a directory and @var{hash} must be the hash of an archive
containing this output.

When @var{references-graphs} is true, it must be a list of file
name/store path pairs.  In that case, the reference graph of each store
path is exported in the build environment in the corresponding file, in
a simple text format.

When @var{allowed-references} is true, it must be a list of store items
or outputs that the derivation's output may refer to.  Likewise,
@var{disallowed-references}, if true, must be a list of things the
outputs may @emph{not} refer to.

When @var{leaked-env-vars} is true, it must be a list of strings
denoting environment variables that are allowed to ``leak'' from the
daemon's environment to the build environment.  This is only applicable
to fixed-output derivations---i.e., when @var{hash} is true.  The main
use is to allow variables such as @code{http_proxy} to be passed to
derivations that download files.

When @var{local-build?} is true, declare that the derivation is not a
good candidate for offloading and should rather be built locally
(@pxref{Daemon Offload Setup}).  This is the case for small derivations
where the costs of data transfers would outweigh the benefits.

When @var{substitutable?} is false, declare that substitutes of the
derivation's output should not be used (@pxref{Substitutes}).  This is
useful, for instance, when building packages that capture details of the
host CPU instruction set.
@end deffn

@noindent
Here's an example with a shell script as its builder, assuming
@var{store} is an open connection to the daemon, and @var{bash} points
to a Bash executable in the store:

@lisp
(use-modules (guix utils)
             (guix store)
             (guix derivations))

(let ((builder   ; add the Bash script to the store
        (add-text-to-store store "my-builder.sh"
                           "echo hello world > $out\n" '())))
  (derivation store "foo"
              bash `("-e" ,builder)
              #:inputs `((,bash) (,builder))
              #:env-vars '(("HOME" . "/homeless"))))
@result{} #<derivation /gnu/store/@dots{}-foo.drv => /gnu/store/@dots{}-foo>
@end lisp

As can be guessed, this primitive is cumbersome to use directly.  A
better approach is to write build scripts in Scheme, of course!  The
best course of action for that is to write the build code as a
``G-expression'', and to pass it to @code{gexp->derivation}.  For more
information, @pxref{G-Expressions}.

Once upon a time, @code{gexp->derivation} did not exist and constructing
derivations with build code written in Scheme was achieved with
@code{build-expression->derivation}, documented below.  This procedure
is now deprecated in favor of the much nicer @code{gexp->derivation}.

@deffn {Scheme Procedure} build-expression->derivation @var{store} @
       @var{name} @var{exp} @
       [#:system (%current-system)] [#:inputs '()] @
       [#:outputs '("out")] [#:hash #f] [#:hash-algo #f] @
       [#:recursive? #f] [#:env-vars '()] [#:modules '()] @
       [#:references-graphs #f] [#:allowed-references #f] @
       [#:disallowed-references #f] @
       [#:local-build? #f] [#:substitutable? #t] [#:guile-for-build #f]
Return a derivation that executes Scheme expression @var{exp} as a
builder for derivation @var{name}.  @var{inputs} must be a list of
@code{(name drv-path sub-drv)} tuples; when @var{sub-drv} is omitted,
@code{"out"} is assumed.  @var{modules} is a list of names of Guile
modules from the current search path to be copied in the store,
compiled, and made available in the load path during the execution of
@var{exp}---e.g., @code{((guix build utils) (guix build
gnu-build-system))}.

@var{exp} is evaluated in an environment where @code{%outputs} is bound
to a list of output/path pairs, and where @code{%build-inputs} is bound
to a list of string/output-path pairs made from @var{inputs}.
Optionally, @var{env-vars} is a list of string pairs specifying the name
and value of environment variables visible to the builder.  The builder
terminates by passing the result of @var{exp} to @code{exit}; thus, when
@var{exp} returns @code{#f}, the build is considered to have failed.

@var{exp} is built using @var{guile-for-build} (a derivation).  When
@var{guile-for-build} is omitted or is @code{#f}, the value of the
@code{%guile-for-build} fluid is used instead.

See the @code{derivation} procedure for the meaning of
@var{references-graphs}, @var{allowed-references},
@var{disallowed-references}, @var{local-build?}, and
@var{substitutable?}.
@end deffn

@noindent
Here's an example of a single-output derivation that creates a directory
containing one file:

@lisp
(let ((builder '(let ((out (assoc-ref %outputs "out")))
                  (mkdir out)    ; create /gnu/store/@dots{}-goo
                  (call-with-output-file (string-append out "/test")
                    (lambda (p)
                      (display '(hello guix) p))))))
  (build-expression->derivation store "goo" builder))

@result{} #<derivation /gnu/store/@dots{}-goo.drv => @dots{}>
@end lisp


@node The Store Monad
@section The Store Monad

@cindex monad

The procedures that operate on the store described in the previous
sections all take an open connection to the build daemon as their first
argument.  Although the underlying model is functional, they either have
side effects or depend on the current state of the store.

The former is inconvenient: the connection to the build daemon has to be
carried around in all those functions, making it impossible to compose
functions that do not take that parameter with functions that do.  The
latter can be problematic: since store operations have side effects
and/or depend on external state, they have to be properly sequenced.

@cindex monadic values
@cindex monadic functions
This is where the @code{(guix monads)} module comes in.  This module
provides a framework for working with @dfn{monads}, and a particularly
useful monad for our uses, the @dfn{store monad}.  Monads are a
construct that allows two things: associating ``context'' with values
(in our case, the context is the store), and building sequences of
computations (here computations include accesses to the store).  Values
in a monad---values that carry this additional context---are called
@dfn{monadic values}; procedures that return such values are called
@dfn{monadic procedures}.

Consider this ``normal'' procedure:

@example
(define (sh-symlink store)
  ;; Return a derivation that symlinks the 'bash' executable.
  (let* ((drv (package-derivation store bash))
         (out (derivation->output-path drv))
         (sh  (string-append out "/bin/bash")))
    (build-expression->derivation store "sh"
                                  `(symlink ,sh %output))))
@end example

Using @code{(guix monads)} and @code{(guix gexp)}, it may be rewritten
as a monadic function:

@example
(define (sh-symlink)
  ;; Same, but return a monadic value.
  (mlet %store-monad ((drv (package->derivation bash)))
    (gexp->derivation "sh"
                      #~(symlink (string-append #$drv "/bin/bash")
                                 #$output))))
@end example

There are several things to note in the second version: the @code{store}
parameter is now implicit and is ``threaded'' in the calls to the
@code{package->derivation} and @code{gexp->derivation} monadic
procedures, and the monadic value returned by @code{package->derivation}
is @dfn{bound} using @code{mlet} instead of plain @code{let}.

As it turns out, the call to @code{package->derivation} can even be
omitted since it will take place implicitly, as we will see later
(@pxref{G-Expressions}):

@example
(define (sh-symlink)
  (gexp->derivation "sh"
                    #~(symlink (string-append #$bash "/bin/bash")
                               #$output)))
@end example

@c See
@c <https://syntaxexclamation.wordpress.com/2014/06/26/escaping-continuations/>
@c for the funny quote.
Calling the monadic @code{sh-symlink} has no effect.  As someone once
said, ``you exit a monad like you exit a building on fire: by running''.
So, to exit the monad and get the desired effect, one must use
@code{run-with-store}:

@example
(run-with-store (open-connection) (sh-symlink))
@result{} /gnu/store/...-sh-symlink
@end example

Note that the @code{(guix monad-repl)} module extends the Guile REPL with
new ``meta-commands'' to make it easier to deal with monadic procedures:
@code{run-in-store}, and @code{enter-store-monad}.  The former is used
to ``run'' a single monadic value through the store:

@example
scheme@@(guile-user)> ,run-in-store (package->derivation hello)
$1 = #<derivation /gnu/store/@dots{}-hello-2.9.drv => @dots{}>
@end example

The latter enters a recursive REPL, where all the return values are
automatically run through the store:

@example
scheme@@(guile-user)> ,enter-store-monad
store-monad@@(guile-user) [1]> (package->derivation hello)
$2 = #<derivation /gnu/store/@dots{}-hello-2.9.drv => @dots{}>
store-monad@@(guile-user) [1]> (text-file "foo" "Hello!")
$3 = "/gnu/store/@dots{}-foo"
store-monad@@(guile-user) [1]> ,q
scheme@@(guile-user)>
@end example

@noindent
Note that non-monadic values cannot be returned in the
@code{store-monad} REPL.

The main syntactic forms to deal with monads in general are provided by
the @code{(guix monads)} module and are described below.

@deffn {Scheme Syntax} with-monad @var{monad} @var{body} ...
Evaluate any @code{>>=} or @code{return} forms in @var{body} as being
in @var{monad}.
@end deffn

@deffn {Scheme Syntax} return @var{val}
Return a monadic value that encapsulates @var{val}.
@end deffn

@deffn {Scheme Syntax} >>= @var{mval} @var{mproc} ...
@dfn{Bind} monadic value @var{mval}, passing its ``contents'' to monadic
procedures @var{mproc}@dots{}@footnote{This operation is commonly
referred to as ``bind'', but that name denotes an unrelated procedure in
Guile.  Thus we use this somewhat cryptic symbol inherited from the
Haskell language.}.  There can be one @var{mproc} or several of them, as
in this example:

@example
(run-with-state
    (with-monad %state-monad
      (>>= (return 1)
           (lambda (x) (return (+ 1 x)))
           (lambda (x) (return (* 2 x)))))
  'some-state)

@result{} 4
@result{} some-state
@end example
@end deffn

@deffn {Scheme Syntax} mlet @var{monad} ((@var{var} @var{mval}) ...) @
       @var{body} ...
@deffnx {Scheme Syntax} mlet* @var{monad} ((@var{var} @var{mval}) ...) @
       @var{body} ...
Bind the variables @var{var} to the monadic values @var{mval} in
@var{body}, which is a sequence of expressions.  As with the bind
operator, this can be thought of as ``unpacking'' the raw, non-monadic
value ``contained'' in @var{mval} and making @var{var} refer to that
raw, non-monadic value within the scope of the @var{body}.  The form
(@var{var} -> @var{val}) binds @var{var} to the ``normal'' value
@var{val}, as per @code{let}.  The binding operations occur in sequence
from left to right.  The last expression of @var{body} must be a monadic
expression, and its result will become the result of the @code{mlet} or
@code{mlet*} when run in the @var{monad}.

@code{mlet*} is to @code{mlet} what @code{let*} is to @code{let}
(@pxref{Local Bindings,,, guile, GNU Guile Reference Manual}).
@end deffn

@deffn {Scheme System} mbegin @var{monad} @var{mexp} ...
Bind @var{mexp} and the following monadic expressions in sequence,
returning the result of the last expression.  Every expression in the
sequence must be a monadic expression.

This is akin to @code{mlet}, except that the return values of the
monadic expressions are ignored.  In that sense, it is analogous to
@code{begin}, but applied to monadic expressions.
@end deffn

@deffn {Scheme System} mwhen @var{condition} @var{mexp0} @var{mexp*} ...
When @var{condition} is true, evaluate the sequence of monadic
expressions @var{mexp0}..@var{mexp*} as in an @code{mbegin}.  When
@var{condition} is false, return @code{*unspecified*} in the current
monad.  Every expression in the sequence must be a monadic expression.
@end deffn

@deffn {Scheme System} munless @var{condition} @var{mexp0} @var{mexp*} ...
When @var{condition} is false, evaluate the sequence of monadic
expressions @var{mexp0}..@var{mexp*} as in an @code{mbegin}.  When
@var{condition} is true, return @code{*unspecified*} in the current
monad.  Every expression in the sequence must be a monadic expression.
@end deffn

@cindex state monad
The @code{(guix monads)} module provides the @dfn{state monad}, which
allows an additional value---the state---to be @emph{threaded} through
monadic procedure calls.

@defvr {Scheme Variable} %state-monad
The state monad.  Procedures in the state monad can access and change
the state that is threaded.

Consider the example below.  The @code{square} procedure returns a value
in the state monad.  It returns the square of its argument, but also
increments the current state value:

@example
(define (square x)
  (mlet %state-monad ((count (current-state)))
    (mbegin %state-monad
      (set-current-state (+ 1 count))
      (return (* x x)))))

(run-with-state (sequence %state-monad (map square (iota 3))) 0)
@result{} (0 1 4)
@result{} 3
@end example

When ``run'' through @var{%state-monad}, we obtain that additional state
value, which is the number of @code{square} calls.
@end defvr

@deffn {Monadic Procedure} current-state
Return the current state as a monadic value.
@end deffn

@deffn {Monadic Procedure} set-current-state @var{value}
Set the current state to @var{value} and return the previous state as a
monadic value.
@end deffn

@deffn {Monadic Procedure} state-push @var{value}
Push @var{value} to the current state, which is assumed to be a list,
and return the previous state as a monadic value.
@end deffn

@deffn {Monadic Procedure} state-pop
Pop a value from the current state and return it as a monadic value.
The state is assumed to be a list.
@end deffn

@deffn {Scheme Procedure} run-with-state @var{mval} [@var{state}]
Run monadic value @var{mval} starting with @var{state} as the initial
state.  Return two values: the resulting value, and the resulting state.
@end deffn

The main interface to the store monad, provided by the @code{(guix
store)} module, is as follows.

@defvr {Scheme Variable} %store-monad
The store monad---an alias for @var{%state-monad}.

Values in the store monad encapsulate accesses to the store.  When its
effect is needed, a value of the store monad must be ``evaluated'' by
passing it to the @code{run-with-store} procedure (see below.)
@end defvr

@deffn {Scheme Procedure} run-with-store @var{store} @var{mval} [#:guile-for-build] [#:system (%current-system)]
Run @var{mval}, a monadic value in the store monad, in @var{store}, an
open store connection.
@end deffn

@deffn {Monadic Procedure} text-file @var{name} @var{text} [@var{references}]
Return as a monadic value the absolute file name in the store of the file
containing @var{text}, a string.  @var{references} is a list of store items that the
resulting text file refers to; it defaults to the empty list.
@end deffn

@deffn {Monadic Procedure} interned-file @var{file} [@var{name}] @
         [#:recursive? #t] [#:select? (const #t)]
Return the name of @var{file} once interned in the store.  Use
@var{name} as its store name, or the basename of @var{file} if
@var{name} is omitted.

When @var{recursive?} is true, the contents of @var{file} are added
recursively; if @var{file} designates a flat file and @var{recursive?}
is true, its contents are added, and its permission bits are kept.

When @var{recursive?} is true, call @code{(@var{select?} @var{file}
@var{stat})} for each directory entry, where @var{file} is the entry's
absolute file name and @var{stat} is the result of @code{lstat}; exclude
entries for which @var{select?} does not return true.

The example below adds a file to the store, under two different names:

@example
(run-with-store (open-connection)
  (mlet %store-monad ((a (interned-file "README"))
                      (b (interned-file "README" "LEGU-MIN")))
    (return (list a b))))

@result{} ("/gnu/store/rwm@dots{}-README" "/gnu/store/44i@dots{}-LEGU-MIN")
@end example

@end deffn

The @code{(guix packages)} module exports the following package-related
monadic procedures:

@deffn {Monadic Procedure} package-file @var{package} [@var{file}] @
       [#:system (%current-system)] [#:target #f] @
       [#:output "out"]
Return as a monadic
value in the absolute file name of @var{file} within the @var{output}
directory of @var{package}.  When @var{file} is omitted, return the name
of the @var{output} directory of @var{package}.  When @var{target} is
true, use it as a cross-compilation target triplet.
@end deffn

@deffn {Monadic Procedure} package->derivation @var{package} [@var{system}]
@deffnx {Monadic Procedure} package->cross-derivation @var{package} @
          @var{target} [@var{system}]
Monadic version of @code{package-derivation} and
@code{package-cross-derivation} (@pxref{Defining Packages}).
@end deffn


@node G-Expressions
@section G-Expressions

@cindex G-expression
@cindex build code quoting
So we have ``derivations'', which represent a sequence of build actions
to be performed to produce an item in the store (@pxref{Derivations}).
These build actions are performed when asking the daemon to actually
build the derivations; they are run by the daemon in a container
(@pxref{Invoking guix-daemon}).

@cindex strata of code
It should come as no surprise that we like to write these build actions
in Scheme.  When we do that, we end up with two @dfn{strata} of Scheme
code@footnote{The term @dfn{stratum} in this context was coined by
Manuel Serrano et al.@: in the context of their work on Hop.  Oleg
Kiselyov, who has written insightful
@url{http://okmij.org/ftp/meta-programming/#meta-scheme, essays and code
on this topic}, refers to this kind of code generation as
@dfn{staging}.}: the ``host code''---code that defines packages, talks
to the daemon, etc.---and the ``build code''---code that actually
performs build actions, such as making directories, invoking
@command{make}, etc.

To describe a derivation and its build actions, one typically needs to
embed build code inside host code.  It boils down to manipulating build
code as data, and the homoiconicity of Scheme---code has a direct
representation as data---comes in handy for that.  But we need more than
the normal @code{quasiquote} mechanism in Scheme to construct build
expressions.

The @code{(guix gexp)} module implements @dfn{G-expressions}, a form of
S-expressions adapted to build expressions.  G-expressions, or
@dfn{gexps}, consist essentially of three syntactic forms: @code{gexp},
@code{ungexp}, and @code{ungexp-splicing} (or simply: @code{#~},
@code{#$}, and @code{#$@@}), which are comparable to
@code{quasiquote}, @code{unquote}, and @code{unquote-splicing},
respectively (@pxref{Expression Syntax, @code{quasiquote},, guile,
GNU Guile Reference Manual}).  However, there are major differences:

@itemize
@item
Gexps are meant to be written to a file and run or manipulated by other
processes.

@item
When a high-level object such as a package or derivation is unquoted
inside a gexp, the result is as if its output file name had been
introduced.

@item
Gexps carry information about the packages or derivations they refer to,
and these dependencies are automatically added as inputs to the build
processes that use them.
@end itemize

@cindex lowering, of high-level objects in gexps
This mechanism is not limited to package and derivation
objects: @dfn{compilers} able to ``lower'' other high-level objects to
derivations or files in the store can be defined,
such that these objects can also be inserted
into gexps.  For example, a useful type of high-level objects that can be
inserted in a gexp is ``file-like objects'', which make it easy to
add files to the store and to refer to them in
derivations and such (see @code{local-file} and @code{plain-file}
below.)

To illustrate the idea, here is an example of a gexp:

@example
(define build-exp
  #~(begin
      (mkdir #$output)
      (chdir #$output)
      (symlink (string-append #$coreutils "/bin/ls")
               "list-files")))
@end example

This gexp can be passed to @code{gexp->derivation}; we obtain a
derivation that builds a directory containing exactly one symlink to
@file{/gnu/store/@dots{}-coreutils-8.22/bin/ls}:

@example
(gexp->derivation "the-thing" build-exp)
@end example

As one would expect, the @code{"/gnu/store/@dots{}-coreutils-8.22"} string is
substituted to the reference to the @var{coreutils} package in the
actual build code, and @var{coreutils} is automatically made an input to
the derivation.  Likewise, @code{#$output} (equivalent to @code{(ungexp
output)}) is replaced by a string containing the directory name of the
output of the derivation.

@cindex cross compilation
In a cross-compilation context, it is useful to distinguish between
references to the @emph{native} build of a package---that can run on the
host---versus references to cross builds of a package.  To that end, the
@code{#+} plays the same role as @code{#$}, but is a reference to a
native package build:

@example
(gexp->derivation "vi"
   #~(begin
       (mkdir #$output)
       (system* (string-append #+coreutils "/bin/ln")
                "-s"
                (string-append #$emacs "/bin/emacs")
                (string-append #$output "/bin/vi")))
   #:target "mips64el-linux-gnu")
@end example

@noindent
In the example above, the native build of @var{coreutils} is used, so
that @command{ln} can actually run on the host; but then the
cross-compiled build of @var{emacs} is referenced.

@cindex imported modules, for gexps
@findex with-imported-modules
Another gexp feature is @dfn{imported modules}: sometimes you want to be
able to use certain Guile modules from the ``host environment'' in the
gexp, so those modules should be imported in the ``build environment''.
The @code{with-imported-modules} form allows you to express that:

@example
(let ((build (with-imported-modules '((guix build utils))
               #~(begin
                   (use-modules (guix build utils))
                   (mkdir-p (string-append #$output "/bin"))))))
  (gexp->derivation "empty-dir"
                    #~(begin
                        #$build
                        (display "success!\n")
                        #t)))
@end example

@noindent
In this example, the @code{(guix build utils)} module is automatically
pulled into the isolated build environment of our gexp, such that
@code{(use-modules (guix build utils))} works as expected.

@cindex module closure
@findex source-module-closure
Usually you want the @emph{closure} of the module to be imported---i.e.,
the module itself and all the modules it depends on---rather than just
the module; failing to do that, attempts to use the module will fail
because of missing dependent modules.  The @code{source-module-closure}
procedure computes the closure of a module by looking at its source file
headers, which comes in handy in this case:

@example
(use-modules (guix modules))   ;for 'source-module-closure'

(with-imported-modules (source-module-closure
                         '((guix build utils)
                           (gnu build vm)))
  (gexp->derivation "something-with-vms"
                    #~(begin
                        (use-modules (guix build utils)
                                     (gnu build vm))
                        @dots{})))
@end example

The syntactic form to construct gexps is summarized below.

@deffn {Scheme Syntax} #~@var{exp}
@deffnx {Scheme Syntax} (gexp @var{exp})
Return a G-expression containing @var{exp}.  @var{exp} may contain one
or more of the following forms:

@table @code
@item #$@var{obj}
@itemx (ungexp @var{obj})
Introduce a reference to @var{obj}.  @var{obj} may have one of the
supported types, for example a package or a
derivation, in which case the @code{ungexp} form is replaced by its
output file name---e.g., @code{"/gnu/store/@dots{}-coreutils-8.22}.

If @var{obj} is a list, it is traversed and references to supported
objects are substituted similarly.

If @var{obj} is another gexp, its contents are inserted and its
dependencies are added to those of the containing gexp.

If @var{obj} is another kind of object, it is inserted as is.

@item #$@var{obj}:@var{output}
@itemx (ungexp @var{obj} @var{output})
This is like the form above, but referring explicitly to the
@var{output} of @var{obj}---this is useful when @var{obj} produces
multiple outputs (@pxref{Packages with Multiple Outputs}).

@item #+@var{obj}
@itemx #+@var{obj}:output
@itemx (ungexp-native @var{obj})
@itemx (ungexp-native @var{obj} @var{output})
Same as @code{ungexp}, but produces a reference to the @emph{native}
build of @var{obj} when used in a cross compilation context.

@item #$output[:@var{output}]
@itemx (ungexp output [@var{output}])
Insert a reference to derivation output @var{output}, or to the main
output when @var{output} is omitted.

This only makes sense for gexps passed to @code{gexp->derivation}.

@item #$@@@var{lst}
@itemx (ungexp-splicing @var{lst})
Like the above, but splices the contents of @var{lst} inside the
containing list.

@item #+@@@var{lst}
@itemx (ungexp-native-splicing @var{lst})
Like the above, but refers to native builds of the objects listed in
@var{lst}.

@end table

G-expressions created by @code{gexp} or @code{#~} are run-time objects
of the @code{gexp?} type (see below.)
@end deffn

@deffn {Scheme Syntax} with-imported-modules @var{modules} @var{body}@dots{}
Mark the gexps defined in @var{body}@dots{} as requiring @var{modules}
in their execution environment.

Each item in @var{modules} can be the name of a module, such as
@code{(guix build utils)}, or it can be a module name, followed by an
arrow, followed by a file-like object:

@example
`((guix build utils)
  (guix gcrypt)
  ((guix config) => ,(scheme-file "config.scm"
                                  #~(define-module @dots{}))))
@end example

@noindent
In the example above, the first two modules are taken from the search
path, and the last one is created from the given file-like object.

This form has @emph{lexical} scope: it has an effect on the gexps
directly defined in @var{body}@dots{}, but not on those defined, say, in
procedures called from @var{body}@dots{}.
@end deffn

@deffn {Scheme Procedure} gexp? @var{obj}
Return @code{#t} if @var{obj} is a G-expression.
@end deffn

G-expressions are meant to be written to disk, either as code building
some derivation, or as plain files in the store.  The monadic procedures
below allow you to do that (@pxref{The Store Monad}, for more
information about monads.)

@deffn {Monadic Procedure} gexp->derivation @var{name} @var{exp} @
       [#:system (%current-system)] [#:target #f] [#:graft? #t] @
       [#:hash #f] [#:hash-algo #f] @
       [#:recursive? #f] [#:env-vars '()] [#:modules '()] @
       [#:module-path @var{%load-path}] @
       [#:references-graphs #f] [#:allowed-references #f] @
       [#:disallowed-references #f] @
       [#:leaked-env-vars #f] @
       [#:script-name (string-append @var{name} "-builder")] @
       [#:local-build? #f] [#:substitutable? #t] [#:guile-for-build #f]
Return a derivation @var{name} that runs @var{exp} (a gexp) with
@var{guile-for-build} (a derivation) on @var{system}; @var{exp} is
stored in a file called @var{script-name}.  When @var{target} is true,
it is used as the cross-compilation target triplet for packages referred
to by @var{exp}.

@var{modules} is deprecated in favor of @code{with-imported-modules}.
Its meaning is to
make @var{modules} available in the evaluation context of @var{exp};
@var{modules} is a list of names of Guile modules searched in
@var{module-path} to be copied in the store, compiled, and made available in
the load path during the execution of @var{exp}---e.g., @code{((guix
build utils) (guix build gnu-build-system))}.

@var{graft?} determines whether packages referred to by @var{exp} should be grafted when
applicable.

When @var{references-graphs} is true, it must be a list of tuples of one of the
following forms:

@example
(@var{file-name} @var{package})
(@var{file-name} @var{package} @var{output})
(@var{file-name} @var{derivation})
(@var{file-name} @var{derivation} @var{output})
(@var{file-name} @var{store-item})
@end example

The right-hand-side of each element of @var{references-graphs} is automatically made
an input of the build process of @var{exp}.  In the build environment, each
@var{file-name} contains the reference graph of the corresponding item, in a simple
text format.

@var{allowed-references} must be either @code{#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 @var{disallowed-references}, which can list items that must not be
referenced by the outputs.

The other arguments are as for @code{derivation} (@pxref{Derivations}).
@end deffn

@cindex file-like objects
The @code{local-file}, @code{plain-file}, @code{computed-file},
@code{program-file}, and @code{scheme-file} procedures below return
@dfn{file-like objects}.  That is, when unquoted in a G-expression,
these objects lead to a file in the store.  Consider this G-expression:

@example
#~(system* #$(file-append glibc "/sbin/nscd") "-f"
           #$(local-file "/tmp/my-nscd.conf"))
@end example

The effect here is to ``intern'' @file{/tmp/my-nscd.conf} by copying it
to the store.  Once expanded, for instance @i{via}
@code{gexp->derivation}, the G-expression refers to that copy under
@file{/gnu/store}; thus, modifying or removing the file in @file{/tmp}
does not have any effect on what the G-expression does.
@code{plain-file} can be used similarly; it differs in that the file
content is directly passed as a string.

@deffn {Scheme Procedure} local-file @var{file} [@var{name}] @
   [#:recursive? #f] [#:select? (const #t)]
Return an object representing local file @var{file} to add to the store; this
object can be used in a gexp.  If @var{file} is a relative file name, it is looked
up relative to the source file where this form appears.  @var{file} will be added to
the store under @var{name}--by default the base name of @var{file}.

When @var{recursive?} is true, the contents of @var{file} are added recursively; if @var{file}
designates a flat file and @var{recursive?} is true, its contents are added, and its
permission bits are kept.

When @var{recursive?} is true, call @code{(@var{select?} @var{file}
@var{stat})} for each directory entry, where @var{file} is the entry's
absolute file name and @var{stat} is the result of @code{lstat}; exclude
entries for which @var{select?} does not return true.

This is the declarative counterpart of the @code{interned-file} monadic
procedure (@pxref{The Store Monad, @code{interned-file}}).
@end deffn

@deffn {Scheme Procedure} plain-file @var{name} @var{content}
Return an object representing a text file called @var{name} with the given
@var{content} (a string) to be added to the store.

This is the declarative counterpart of @code{text-file}.
@end deffn

@deffn {Scheme Procedure} computed-file @var{name} @var{gexp} @
          [#:options '(#:local-build? #t)]
Return an object representing the store item @var{name}, a file or
directory computed by @var{gexp}.  @var{options}
is a list of additional arguments to pass to @code{gexp->derivation}.

This is the declarative counterpart of @code{gexp->derivation}.
@end deffn

@deffn {Monadic Procedure} gexp->script @var{name} @var{exp}
Return an executable script @var{name} that runs @var{exp} using
@var{guile}, with @var{exp}'s imported modules in its search path.

The example below builds a script that simply invokes the @command{ls}
command:

@example
(use-modules (guix gexp) (gnu packages base))

(gexp->script "list-files"
              #~(execl #$(file-append coreutils "/bin/ls")
                       "ls"))
@end example

When ``running'' it through the store (@pxref{The Store Monad,
@code{run-with-store}}), we obtain a derivation that produces an
executable file @file{/gnu/store/@dots{}-list-files} along these lines:

@example
#!/gnu/store/@dots{}-guile-2.0.11/bin/guile -ds