66 lines
1.9 KiB
Scheme
66 lines
1.9 KiB
Scheme
;;;
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;;; Time-stamp: <2008-06-19 21:11:59 noel>
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;;;
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;;; Copyright (C) 2005 by Noel Welsh.
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;;;
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;;; This library is free software; you can redistribute it
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;;; and/or modify it under the terms of the GNU Lesser
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;;; General Public License as published by the Free Software
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;;; Foundation; either version 2.1 of the License, or (at
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;;; your option) any later version.
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;;; This library is distributed in the hope that it will be
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;;; useful, but WITHOUT ANY WARRANTY; without even the
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;;; implied warranty of MERCHANTABILITY or FITNESS FOR A
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;;; PARTICULAR PURPOSE. See the GNU Lesser General Public
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;;; License for more details.
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;;; You should have received a copy of the GNU Lesser
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;;; General Public License along with this library; if not,
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;;; write to the Free Software Foundation, Inc., 59 Temple
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;;; Place, Suite 330, Boston, MA 02111-1307 USA
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;;; Author: Noel Welsh <noelwelsh@yahoo.com>
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;;
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;;
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;; Commentary:
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#lang scheme/base
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(provide (all-defined-out))
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;; struct monad : any any
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(define-struct monad (value state) #:mutable)
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;; compose : (monad-of 'a) ('a -> (monad-of 'b)) -> (monad-of 'b)
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(define (compose comp build-comp)
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(lambda (seed0)
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(let* ((seed1 (comp seed0))
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(value (monad-value seed1)))
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((build-comp value) seed1))))
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;; compose*: (monad-of 'a) ('a -> (monad-of 'b)) ... -> (monad-of 'b)
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(define (compose* monad . actions)
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(if (null? actions)
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monad
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(compose monad
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(lambda (value)
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(apply
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compose*
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((car actions) value)
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(cdr actions))))))
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;; sequence : (monad-of 'a) (monad-of 'b) -> (monad-of 'b)
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(define (sequence monad-a monad-b)
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(compose monad-a (lambda (v) monad-b)))
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(define (sequence* monad . monads)
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(if (null? monads)
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monad
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(sequence monad
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(apply
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sequence*
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(car monads)
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(cdr monads)))))
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