8bc2183fe5
* doc/guix.texi (The Store Monad) <mlet, mlet*, mbegin>: Clarify their intended usage. * guix/monads.scm (mbegin): Update docstring accordingly. Signed-off-by: Ludovic Courtès <ludo@gnu.org>
417 lines
13 KiB
Scheme
417 lines
13 KiB
Scheme
;;; GNU Guix --- Functional package management for GNU
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;;; Copyright © 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
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;;;
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;;; This file is part of GNU Guix.
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;;;
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;;; GNU Guix is free software; you can redistribute it and/or modify it
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;;; under the terms of the GNU General Public License as published by
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;;; the Free Software Foundation; either version 3 of the License, or (at
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;;; your option) any later version.
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;;;
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;;; GNU Guix is distributed in the hope that it will be useful, but
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;;; WITHOUT ANY WARRANTY; without even the implied warranty of
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;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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;;; GNU General Public License for more details.
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;;;
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;;; You should have received a copy of the GNU General Public License
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;;; along with GNU Guix. If not, see <http://www.gnu.org/licenses/>.
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(define-module (guix monads)
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#:use-module ((system syntax)
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#:select (syntax-local-binding))
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#:use-module (ice-9 match)
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#:use-module (srfi srfi-1)
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#:use-module (srfi srfi-9)
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#:use-module (srfi srfi-26)
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#:export (;; Monads.
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define-monad
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monad?
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monad-bind
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monad-return
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;; Syntax.
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>>=
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return
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with-monad
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mlet
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mlet*
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mbegin
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mwhen
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munless
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lift0 lift1 lift2 lift3 lift4 lift5 lift6 lift7 lift
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listm
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foldm
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mapm
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sequence
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anym
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;; Concrete monads.
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%identity-monad
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%state-monad
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state-return
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state-bind
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current-state
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set-current-state
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state-push
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state-pop
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run-with-state))
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;;; Commentary:
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;;;
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;;; This module implements the general mechanism of monads, and provides in
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;;; particular an instance of the "state" monad. The API was inspired by that
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;;; of Racket's "better-monads" module (see
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;;; <http://planet.racket-lang.org/package-source/toups/functional.plt/1/1/planet-docs/better-monads-guide/index.html>).
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;;; The implementation and use case were influenced by Oleg Kysielov's
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;;; "Monadic Programming in Scheme" (see
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;;; <http://okmij.org/ftp/Scheme/monad-in-Scheme.html>).
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;;;
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;;; Code:
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;; Record type for monads manipulated at run time.
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(define-record-type <monad>
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(make-monad bind return)
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monad?
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(bind monad-bind)
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(return monad-return)) ; TODO: Add 'plus' and 'zero'
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(define-syntax define-monad
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(lambda (s)
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"Define the monad under NAME, with the given bind and return methods."
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(define prefix (string->symbol "% "))
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(define (make-rtd-name name)
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(datum->syntax name
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(symbol-append prefix (syntax->datum name) '-rtd)))
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(syntax-case s (bind return)
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((_ name (bind b) (return r))
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(with-syntax ((rtd (make-rtd-name #'name)))
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#`(begin
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(define rtd
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;; The record type, for use at run time.
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(make-monad b r))
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(define-syntax name
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;; An "inlined record", for use at expansion time. The goal is
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;; to allow 'bind' and 'return' to be resolved at expansion
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;; time, in the common case where the monad is accessed
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;; directly as NAME.
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(lambda (s)
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(syntax-case s (%bind %return)
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((_ %bind) #'b)
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((_ %return) #'r)
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(_ #'rtd))))))))))
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(define-syntax-parameter >>=
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;; The name 'bind' is already taken, so we choose this (obscure) symbol.
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(lambda (s)
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(syntax-violation '>>= ">>= (bind) used outside of 'with-monad'" s)))
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(define-syntax-parameter return
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(lambda (s)
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(syntax-violation 'return "return used outside of 'with-monad'" s)))
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(define-syntax-rule (bind-syntax bind)
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"Return a macro transformer that handles the expansion of '>>=' expressions
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using BIND as the binary bind operator.
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This macro exists to allow the expansion of n-ary '>>=' expressions, even
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though BIND is simply binary, as in:
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(with-monad %state-monad
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(>>= (return 1)
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(lift 1+ %state-monad)
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(lift 1+ %state-monad)))
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"
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(lambda (stx)
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(define (expand body)
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(syntax-case body ()
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((_ mval mproc)
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#'(bind mval mproc))
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((x mval mproc0 mprocs (... ...))
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(expand #'(>>= (>>= mval mproc0)
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mprocs (... ...))))))
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(expand stx)))
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(define-syntax with-monad
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(lambda (s)
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"Evaluate BODY in the context of MONAD, and return its result."
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(syntax-case s ()
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((_ monad body ...)
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(eq? 'macro (syntax-local-binding #'monad))
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;; MONAD is a syntax transformer, so we can obtain the bind and return
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;; methods by directly querying it.
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#'(syntax-parameterize ((>>= (bind-syntax (monad %bind)))
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(return (identifier-syntax (monad %return))))
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body ...))
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((_ monad body ...)
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;; MONAD refers to the <monad> record that represents the monad at run
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;; time, so use the slow method.
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#'(syntax-parameterize ((>>= (bind-syntax
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(monad-bind monad)))
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(return (identifier-syntax
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(monad-return monad))))
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body ...)))))
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(define-syntax mlet*
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(syntax-rules (->)
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"Bind the given monadic values MVAL to the given variables VAR. When the
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form is (VAR -> VAL), bind VAR to the non-monadic value VAL in the same way as
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'let'."
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;; Note: the '->' symbol corresponds to 'is:' in 'better-monads.rkt'.
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((_ monad () body ...)
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(with-monad monad body ...))
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((_ monad ((var mval) rest ...) body ...)
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(with-monad monad
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(>>= mval
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(lambda (var)
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(mlet* monad (rest ...)
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body ...)))))
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((_ monad ((var -> val) rest ...) body ...)
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(let ((var val))
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(mlet* monad (rest ...)
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body ...)))))
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(define-syntax mlet
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(lambda (s)
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(syntax-case s ()
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((_ monad ((var mval ...) ...) body ...)
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(with-syntax (((temp ...) (generate-temporaries #'(var ...))))
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#'(mlet* monad ((temp mval ...) ...)
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(let ((var temp) ...)
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body ...)))))))
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(define-syntax mbegin
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(syntax-rules (%current-monad)
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"Bind MEXP and the following monadic expressions in sequence, returning
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the result of the last expression. Every expression in the sequence must be a
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monadic expression."
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((_ %current-monad mexp)
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mexp)
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((_ %current-monad mexp rest ...)
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(>>= mexp
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(lambda (unused-value)
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(mbegin %current-monad rest ...))))
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((_ monad mexp)
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(with-monad monad
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mexp))
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((_ monad mexp rest ...)
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(with-monad monad
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(>>= mexp
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(lambda (unused-value)
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(mbegin monad rest ...)))))))
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(define-syntax mwhen
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(syntax-rules ()
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"When CONDITION is true, evaluate the sequence of monadic expressions
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MEXP0..MEXP* as in an 'mbegin'. When CONDITION is false, return *unspecified*
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in the current monad. Every expression in the sequence must be a monadic
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expression."
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((_ condition mexp0 mexp* ...)
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(if condition
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(mbegin %current-monad
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mexp0 mexp* ...)
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(return *unspecified*)))))
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(define-syntax munless
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(syntax-rules ()
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"When CONDITION is false, evaluate the sequence of monadic expressions
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MEXP0..MEXP* as in an 'mbegin'. When CONDITION is true, return *unspecified*
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in the current monad. Every expression in the sequence must be a monadic
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expression."
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((_ condition mexp0 mexp* ...)
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(if condition
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(return *unspecified*)
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(mbegin %current-monad
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mexp0 mexp* ...)))))
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(define-syntax define-lift
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(syntax-rules ()
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((_ liftn (args ...))
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(define-syntax liftn
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(lambda (s)
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"Lift PROC to MONAD---i.e., return a monadic function in MONAD."
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(syntax-case s ()
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((liftn proc monad)
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;; Inline the result of lifting PROC, such that 'return' can in
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;; turn be open-coded.
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#'(lambda (args ...)
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(with-monad monad
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(return (proc args ...)))))
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(id
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(identifier? #'id)
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;; Slow path: Return a closure-returning procedure (we don't
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;; guarantee (eq? LIFTN LIFTN), but that's fine.)
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#'(lambda (proc monad)
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(lambda (args ...)
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(with-monad monad
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(return (proc args ...))))))))))))
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(define-lift lift0 ())
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(define-lift lift1 (a))
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(define-lift lift2 (a b))
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(define-lift lift3 (a b c))
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(define-lift lift4 (a b c d))
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(define-lift lift5 (a b c d e))
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(define-lift lift6 (a b c d e f))
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(define-lift lift7 (a b c d e f g))
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(define (lift proc monad)
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"Lift PROC, a procedure that accepts an arbitrary number of arguments, to
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MONAD---i.e., return a monadic function in MONAD."
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(lambda args
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(with-monad monad
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(return (apply proc args)))))
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(define (foldm monad mproc init lst)
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"Fold MPROC over LST and return a monadic value seeded by INIT.
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(foldm %state-monad (lift2 cons %state-monad) '() '(a b c))
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=> '(c b a) ;monadic
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"
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(with-monad monad
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(let loop ((lst lst)
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(result init))
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(match lst
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(()
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(return result))
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((head tail ...)
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(>>= (mproc head result)
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(lambda (result)
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(loop tail result))))))))
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(define (mapm monad mproc lst)
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"Map MPROC over LST and return a monadic list.
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(mapm %state-monad (lift1 1+ %state-monad) '(0 1 2))
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=> (1 2 3) ;monadic
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"
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(mlet monad ((result (foldm monad
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(lambda (item result)
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(>>= (mproc item)
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(lambda (item)
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(return (cons item result)))))
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'()
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lst)))
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(return (reverse result))))
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(define-syntax-rule (sequence monad lst)
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"Turn the list of monadic values LST into a monadic list of values, by
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evaluating each item of LST in sequence."
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;; XXX: Making it a macro is a bit brutal as it leads to a lot of code
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;; duplication. However, it allows >>= and return to be open-coded, which
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;; avoids struct-ref's to MONAD and a few closure allocations when using
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;; %STATE-MONAD.
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(with-monad monad
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(let seq ((lstx lst)
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(result '()))
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(match lstx
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(()
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(return (reverse result)))
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((head . tail)
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(>>= head
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(lambda (item)
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(seq tail (cons item result)))))))))
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(define (anym monad mproc lst)
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"Apply MPROC to the list of values LST; return as a monadic value the first
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value for which MPROC returns a true monadic value or #f. For example:
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(anym %state-monad (lift1 odd? %state-monad) '(0 1 2))
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=> #t ;monadic
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"
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(with-monad monad
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(let loop ((lst lst))
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(match lst
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(()
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(return #f))
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((head tail ...)
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(>>= (mproc head)
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(lambda (result)
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(if result
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(return result)
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(loop tail)))))))))
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(define-syntax listm
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(lambda (s)
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"Return a monadic list in MONAD from the monadic values MVAL."
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(syntax-case s ()
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((_ monad mval ...)
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(with-syntax (((val ...) (generate-temporaries #'(mval ...))))
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#'(mlet monad ((val mval) ...)
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(return (list val ...))))))))
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;;;
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;;; Identity monad.
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;;;
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(define-inlinable (identity-return value)
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value)
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(define-inlinable (identity-bind mvalue mproc)
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(mproc mvalue))
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(define-monad %identity-monad
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(bind identity-bind)
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(return identity-return))
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;;;
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;;; State monad.
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;;;
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(define-inlinable (state-return value)
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(lambda (state)
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(values value state)))
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(define-inlinable (state-bind mvalue mproc)
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"Bind MVALUE, a value in the state monad, and pass it to MPROC."
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(lambda (state)
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(call-with-values
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(lambda ()
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(mvalue state))
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(lambda (value state)
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;; Note: as of Guile 2.0.11, declaring a variable to hold the result
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;; of (mproc value) prevents a bit of unfolding/inlining.
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((mproc value) state)))))
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(define-monad %state-monad
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(bind state-bind)
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(return state-return))
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(define* (run-with-state mval #:optional (state '()))
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"Run monadic value MVAL starting with STATE as the initial state. Return
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two values: the resulting value, and the resulting state."
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(mval state))
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(define-inlinable (current-state)
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"Return the current state as a monadic value."
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(lambda (state)
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(values state state)))
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(define-inlinable (set-current-state value)
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"Set the current state to VALUE and return the previous state as a monadic
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value."
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(lambda (state)
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(values state value)))
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(define (state-pop)
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"Pop a value from the current state and return it as a monadic value. The
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state is assumed to be a list."
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(lambda (state)
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(match state
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((head . tail)
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(values head tail)))))
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(define (state-push value)
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"Push VALUE to the current state, which is assumed to be a list, and return
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the previous state as a monadic value."
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(lambda (state)
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(values state (cons value state))))
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;;; monads.scm end here
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