117 lines
4.2 KiB
Scheme
117 lines
4.2 KiB
Scheme
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;;; GNU Guix --- Functional package management for GNU
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;;; Copyright © 2012, 2013, 2014, 2015, 2016 Ludovic Courtès <ludo@gnu.org>
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;;; Copyright © 2014 Eric Bavier <bavier@member.fsf.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 combinators)
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#:use-module (ice-9 match)
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#:use-module (ice-9 vlist)
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#:export (memoize
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fold2
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fold-tree
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fold-tree-leaves
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compile-time-value))
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;;; Commentary:
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;;;
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;;; This module provides useful combinators that complement SRFI-1 and
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;;; friends.
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;;;
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;;; Code:
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(define (memoize proc)
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"Return a memoizing version of PROC."
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(let ((cache (make-hash-table)))
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(lambda args
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(let ((results (hash-ref cache args)))
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(if results
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(apply values results)
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(let ((results (call-with-values (lambda ()
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(apply proc args))
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list)))
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(hash-set! cache args results)
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(apply values results)))))))
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(define fold2
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(case-lambda
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((proc seed1 seed2 lst)
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"Like `fold', but with a single list and two seeds."
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(let loop ((result1 seed1)
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(result2 seed2)
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(lst lst))
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(if (null? lst)
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(values result1 result2)
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(call-with-values
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(lambda () (proc (car lst) result1 result2))
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(lambda (result1 result2)
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(loop result1 result2 (cdr lst)))))))
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((proc seed1 seed2 lst1 lst2)
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"Like `fold', but with a two lists and two seeds."
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(let loop ((result1 seed1)
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(result2 seed2)
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(lst1 lst1)
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(lst2 lst2))
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(if (or (null? lst1) (null? lst2))
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(values result1 result2)
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(call-with-values
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(lambda () (proc (car lst1) (car lst2) result1 result2))
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(lambda (result1 result2)
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(fold2 proc result1 result2 (cdr lst1) (cdr lst2)))))))))
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(define (fold-tree proc init children roots)
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"Call (PROC NODE RESULT) for each node in the tree that is reachable from
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ROOTS, using INIT as the initial value of RESULT. The order in which nodes
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are traversed is not specified, however, each node is visited only once, based
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on an eq? check. Children of a node to be visited are generated by
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calling (CHILDREN NODE), the result of which should be a list of nodes that
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are connected to NODE in the tree, or '() or #f if NODE is a leaf node."
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(let loop ((result init)
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(seen vlist-null)
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(lst roots))
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(match lst
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(() result)
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((head . tail)
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(if (not (vhash-assq head seen))
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(loop (proc head result)
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(vhash-consq head #t seen)
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(match (children head)
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((or () #f) tail)
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(children (append tail children))))
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(loop result seen tail))))))
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(define (fold-tree-leaves proc init children roots)
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"Like fold-tree, but call (PROC NODE RESULT) only for leaf nodes."
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(fold-tree
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(lambda (node result)
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(match (children node)
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((or () #f) (proc node result))
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(else result)))
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init children roots))
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(define-syntax compile-time-value ;not quite at home
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(syntax-rules ()
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"Evaluate the given expression at compile time. The expression must
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evaluate to a simple datum."
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((_ exp)
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(let-syntax ((v (lambda (s)
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(let ((val exp))
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(syntax-case s ()
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(_ #`'#,(datum->syntax s val)))))))
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v))))
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;;; combinators.scm ends here
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