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.
This is where the
(guix monads) module comes in. This module
provides a framework for working with monads, and a particularly
useful monad for our uses, the 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
monadic values; procedures that return such values are called
Consider this “normal” procedure:
(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))))
(guix monads) and
(guix gexp), it may be rewritten
as a monadic function:
(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))))
There several things to note in the second version: the
parameter is now implicit and is “threaded” in the calls to the
procedures, and the monadic value returned by
is bound using
mlet instead of plain
As it turns out, the call to
package->derivation can even be
omitted since it will take place implicitly, as we will see later
(define (sh-symlink) (gexp->derivation "sh" #~(symlink (string-append #$bash "/bin/bash") #$output)))
Calling the monadic
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
(run-with-store (open-connection) (sh-symlink)) ⇒ /gnu/store/...-sh-symlink
Note that the
(guix monad-repl) module extends the Guile REPL with
new “meta-commands” to make it easier to deal with monadic procedures:
enter-store-monad. The former is used
to “run” a single monadic value through the store:
scheme@(guile-user)> ,run-in-store (package->derivation hello) $1 = #<derivation /gnu/store/…-hello-2.9.drv => …>
The latter enters a recursive REPL, where all the return values are automatically run through the store:
scheme@(guile-user)> ,enter-store-monad store-monad@(guile-user) > (package->derivation hello) $2 = #<derivation /gnu/store/…-hello-2.9.drv => …> store-monad@(guile-user) > (text-file "foo" "Hello!") $3 = "/gnu/store/…-foo" store-monad@(guile-user) > ,q scheme@(guile-user)>
Note that non-monadic values cannot be returned in the
The main syntactic forms to deal with monads in general are provided by
(guix monads) module and are described below.
return forms in body as being
Return a monadic value that encapsulates val.
Bind monadic value mval, passing its “contents” to monadic procedures mproc…7. There can be one mproc or several of them, as in this example:
(run-with-state (with-monad %state-monad (>>= (return 1) (lambda (x) (return (+ 1 x))) (lambda (x) (return (* 2 x))))) 'some-state) ⇒ 4 ⇒ some-state
Bind the variables var to the monadic values mval in
body. The form (var -> val) binds var to the
“normal” value val, as per
mlet* is to
let* is to
(see Local Bindings in GNU Guile Reference Manual).
Bind mexp and the following monadic expressions in sequence, returning the result of the last expression.
This is akin to
mlet, except that the return values of the
monadic expressions are ignored. In that sense, it is analogous to
begin, but applied to monadic expressions.
(guix monads) module provides the state monad, which
allows an additional value—the state—to be threaded through
monadic procedure calls.
The state monad. Procedures in the state monad can access and change the state that is threaded.
Consider the example below. The
square procedure returns a value
in the state monad. It returns the square of its argument, but also
increments the current state value:
(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) ⇒ (0 1 4) ⇒ 3
When “run” through %state-monad, we obtain that additional state
value, which is the number of
Return the current state as a monadic value.
Set the current state to value and return the previous state as a monadic value.
Push value to the current state, which is assumed to be a list, and return the previous state as a monadic value.
Pop a value from the current state and return it as a monadic value. The state is assumed to be a list.
Run monadic value mval starting with state as the initial state. Return two values: the resulting value, and the resulting state.
The main interface to the store monad, provided by the
store) module, is as follows.
The store monad—an alias for %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
run-with-store procedure (see below.)
Run mval, a monadic value in the store monad, in store, an open store connection.
Return as a monadic value the absolute file name in the store of the file containing text, a string. references is a list of store items that the resulting text file refers to; it defaults to the empty list.
Return the name of file once interned in the store. Use name as its store name, or the basename of file if name is omitted.
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.
The example below adds a file to the store, under two different names:
(run-with-store (open-connection) (mlet %store-monad ((a (interned-file "README")) (b (interned-file "README" "LEGU-MIN"))) (return (list a b)))) ⇒ ("/gnu/store/rwm…-README" "/gnu/store/44i…-LEGU-MIN")
(guix packages) module exports the following package-related
Return as a monadic value in the absolute file name of file within the output directory of package. When file is omitted, return the name of the output directory of package. When target is true, use it as a cross-compilation target triplet.
Monadic version of
package-cross-derivation (see Defining Packages).
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.