86 lines
2.0 KiB
Scheme
Executable File
86 lines
2.0 KiB
Scheme
Executable File
(module data-structures (lib "eopl.ss" "eopl")
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(require "lang.scm") ; for expression?
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(provide (all-defined)) ; too many things to list
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;;;;;;;;;;;;;;;; expressed values ;;;;;;;;;;;;;;;;
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;;; an expressed value is either a number, a boolean, a procval, or a
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;;; list of expvals.
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(define-datatype expval expval?
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(num-val
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(value number?))
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(bool-val
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(boolean boolean?))
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(proc-val
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(proc proc?))
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(list-val
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(lst (list-of expval?))))
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;;; extractors:
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(define expval->num
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(lambda (v)
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(cases expval v
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(num-val (num) num)
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(else (expval-extractor-error 'num v)))))
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(define expval->bool
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(lambda (v)
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(cases expval v
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(bool-val (bool) bool)
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(else (expval-extractor-error 'bool v)))))
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(define expval->proc
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(lambda (v)
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(cases expval v
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(proc-val (proc) proc)
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(else (expval-extractor-error 'proc v)))))
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(define expval->list
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(lambda (v)
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(cases expval v
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(list-val (lst) lst)
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(else (expval-extractor-error 'list v)))))
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(define expval-extractor-error
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(lambda (variant value)
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(eopl:error 'expval-extractors "Looking for a ~s, found ~s"
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variant value)))
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;; ;;;;;;;;;;;;;;;; continuations ;;;;;;;;;;;;;;;;
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;; moved to interp.scm
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;;;;;;;;;;;;;;;; procedures ;;;;;;;;;;;;;;;;
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(define-datatype proc proc?
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(procedure
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(bvar symbol?)
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(body expression?)
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(env environment?)))
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;;;;;;;;;;;;;;;; environment structures ;;;;;;;;;;;;;;;;
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;;; replaced by custom environment structure in environments.scm.
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;;; This represents an environment as an alist ((id rhs) ...)
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;;; where rhs is either an expval or a list (bvar body)
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;;; expval is for extend-env; the list is for extend-env-rec.
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;;; this representation is designed to make the printed representation
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;;; of the environment more readable.
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;;; The code for this is in environments.scm, but we need environment?
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;;; for define-datatype proc, so we write an appoximation:
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(define environment?
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(list-of
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(lambda (p)
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(and
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(pair? p)
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(symbol? (car p))))))
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)
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