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exercises: progressive solution for let*

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Nicolas Boulenguez 2019-07-17 11:01:02 +02:00
parent 9c20ca3f39
commit 0f37b1af06
1 changed files with 26 additions and 7 deletions
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33
examples/exercises.mal
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@ -73,18 +73,36 @@
(list 'apply 'list (foldr _quasiquote_iter () ast))
(list 'quote ast)))))
;; FIXME: mutual recursion.
;; http://okmij.org/ftp/Computation/fixed-point-combinators.html
(def! _letA_keys (fn* [binds]
(if (empty? binds)
()
(cons (first binds) (_letA_keys (rest (rest binds)))))))
(def! _letA_values (fn* [binds]
(if (empty? binds)
()
(_letA_keys (rest binds)))))
(def! _letA (fn* [binds form]
(cons (list 'fn* (_letA_keys binds) form) (_letA_values binds))))
;; Fails for (let* [a 1 b (+ 1 a)] b)
(def! _letB (fn* [binds form]
(if (empty? binds)
form
(list (list 'fn* [(first binds)] (_letB (rest (rest binds)) form))
(first (rest binds))))))
;; Fails for (let* (cst (fn* (n) (if (= n 0) nil (cst (- n 1))))) (cst 1))
(def! _c_combinator (fn* [x] (x x)))
(def! _d_combinator (fn* [f] (fn* [x] (f (fn* [v] ((x x) v))))))
(defmacro! let*
(def! _Y_combinator (fn* [x] (_c_combinator (_d_combinator x))))
(def! _letC
(fn* [binds form]
(if (empty? binds)
form
(list (list 'fn* [(first binds)] (list 'let* (rest (rest binds)) form))
(list '_c_combinator
(list '_d_combinator
(list 'fn* [(first binds)] (first (rest binds)))))))))
(list (list 'fn* [(first binds)] (_letC (rest (rest binds)) form))
(list '_Y_combinator (list 'fn* [(first binds)] (first (rest binds))))))))
;; Fails for mutual recursion.
;; See http://okmij.org/ftp/Computation/fixed-point-combinators.html
;; if you are motivated to implement solution D.
(defmacro! let* _letC)
(def! apply
;; Replace (f a b [c d]) with ('f 'a 'b 'c 'd) then evaluate the
@ -148,4 +166,5 @@
;; This `nil` is intentional so that the result of doing `load-file` is
;; `nil` instead of whatever happens to be the last definiton.
;; FIXME: can be removed after merge of load-file-trailing-new-line-nil
nil