define
directly instead of a lambda
that is then named. We used the lambda form just to emphasize what is going on, and that a function is just like any other value. So, normall use define, as in the second form of each example below: (define f (lambda (x y) (+ x y)))
(define (f x y) (+ x y))
(define f (lambda args (print args)))
(define (f1 . args) (print args))
(define g (lambda (x y . args) (list (list x) (list y) (list args))))
(define (g1 x y . args) (list (list x) (list y) (list args)))
if
statements into cond
statements, and all let
statements into applications of a lambda
(as per the examples in the notes). #lang racket |
|
(define (transform-if-to-cond code) |
; locates every occurrence of an if expression in code and |
; transforms it into an equivalent cond |
|
; (if c p q) => (cond (c p) (else q )) |
|
(define (trans c p q) |
(list 'cond (list c p) (list 'else q) ) ) |
|
(if (and (list? code) (not (null? code)) (eq? 'if (first code))) |
|
; recursively transform the parts of the if before transforming the if |
(apply trans (map transform-if-to-cond (rest code))) |
|
(if (list? code) |
; recursively transform expressions |
(map transform-if-to-cond code) |
; leaf expression, leave alone |
code) |
)) |
|
(define (transform-let-to-apply code) |
; locates every occurrence of an let expression in code and |
; transforms it into an equivalent lambda applied to the defining expresions. |
|
; (let ( (v1 e1) (v2 e2) ... (vn en) ) b1 b2 ... bk ) |
; => |
; ((lambda (v1 v2 ... vn) b1 b2 ... bk) e1 e2 ... en) |
|
) |
transform-if-to-cond
function is implemented, but the transform-let-into-apply
is not. (define code-1 |
'(let ( (x 1) (y 2) ) |
;(displayln (list "got" x y)) |
(let ( (z (+ x y)) ) |
;(displayln (list "z" z)) |
(if (< x y) |
(list "x smaller") |
(list "y smaller") |
) |
) |
)) |
|
(define (test-1) |
(newline) |
(displayln code-1) |
(displayln (eval code-1)) |
(newline) |
|
; test the if transform |
(define t1 (transform-if-to-cond code-1)) |
(displayln t1) |
(displayln (eval t1)) |
(newline) |
|
; test the let transform |
(define t2 (transform-let-to-apply code-1)) |
(displayln t2) |
(displayln (eval t2)) |
(newline) |
|
; test the two different composition orders of the transforms |
(define t3 (transform-let-to-apply (transform-if-to-cond code-1))) |
(displayln t3) |
(displayln (eval t3)) |
(newline) |
(define t4 (transform-if-to-cond(transform-let-to-apply code-1))) |
(displayln t4) |
(displayln (eval t4)) |
) |
> (test-1) |
|
'(let ((x 1) (y 2)) (let ((z (+ x y))) (if (< x y) (list "x smaller") (list "y smaller")))) |
'("x smaller") |
|
'(let ((x 1) (y 2)) (let ((z (+ x y))) (cond ((< x y) (list "x smaller")) (else (list "y smaller"))))) |
'("x smaller") |
|
'((lambda (x y) ((lambda (z) (if (< x y) (list "x smaller") (list "y smaller"))) (+ x y))) 1 2) |
'("x smaller") |
|
'((lambda (x y) ((lambda (z) (cond ((< x y) (list "x smaller")) (else (list "y smaller")))) (+ x y))) 1 2) |
'("x smaller") |
|
'((lambda (x y) ((lambda (z) (cond ((< x y) (list "x smaller")) (else (list "y smaller")))) (+ x y))) 1 2) |
'("x smaller") |
transform-let-to-apply
. -- data types are defined by constructor patterns |
-- Eq equality can be derived from structural equality |
|
-- a binary tree consisting of leaves L or internal nodes N |
data BinTree = L | N BinTree BinTree deriving (Eq, Show) |
|
-- this function creates the full binary tree of size 2^(n+1) -1 |
makeBinTree 0 = L |
makeBinTree n = N (makeBinTree (n-1)) (makeBinTree (n-1)) |
|
-- this function computes the size of a binary tree |
size L = 1 |
size (N t1 t2) = 1 + size t1 + size t2 |
$ ghci |
|
GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help |
Loading package ghc-prim ... linking ... done. |
Loading package integer-gmp ... linking ... done. |
Loading package base ... linking ... done. |
Prelude> load assign-2-tree.hs |
|
Prelude> :load assign-2-tree.hs |
[1 of 1] Compiling Main ( assign-2-tree.hs, interpreted ) |
Ok, modules loaded: Main. |
|
*Main> makeBinTree 3 |
N (N (N L L) (N L L)) (N (N L L) (N L L)) |
|
*Main> let t1 = makeBinTree 3 |
|
*Main> t1 |
N (N (N L L) (N L L)) (N (N L L) (N L L)) |
|
*Main> size t1 |
15 |
BinTree
, each internal N
node must have two sub trees. Thus there is no way to construct a tree of size 2. If you want a binary tree that can be of any size, then you need to introduce a new constructor N1
to BinTree
that allows an internal node with only one subtree. Make this modification to BinTree
.
size
to account for this modification.
depth
that gives the depth of the modified BinTree
tree. The depth of a tree of size 1 is 1.
makeABinTree
such that makeABinTree s
constructs a BinTree
of size exactly s
and that has minimum possible depth. I.e. a long stringy tree is not allowed. #lang racket |
|
|
(define (transform-if-to-cond code) |
; locates every occurrence of an if expression in code and |
; transforms it into an equivalent cond |
|
; (if c p q) => (cond (c p) (else q )) |
|
(define (trans c p q) |
(list 'cond (list c p) (list 'else q) ) ) |
|
(if (and (list? code) (not (null? code)) (eq? 'if (first code))) |
|
; recursively transform the parts of the if before transforming the if |
(apply trans (map transform-if-to-cond (rest code))) |
|
(if (list? code) |
; recursively transform expressions |
(map transform-if-to-cond code) |
; leaf expression, leave alone |
code) |
)) |
|
(define (transform-let-to-apply code) |
; locates every occurrence of an let expression in code and |
; transforms it into an equivalent lambda applied to the values. |
|
; (let ( (v1 e1) (v2 e2) ... (vn en) ) b1 b2 ... bk ) |
; => |
; ((lambda (v1 v2 ... vn) b1 b2 ... bk) e1 e2 ... en) |
|
; vars: list of variables being defined (v1 v2 ... vn) |
; exprs list of defining expressions (e1 e2 ... en) to evaluate |
; body: list of code inside the let (b1 b2 ... bk) |
|
(define (trans vars exprs body) |
(append (list (append (list 'lambda vars) body)) exprs) |
) |
|
(if (and (list? code) (not (null? code)) (eq? 'let (first code))) |
|
(let ( (vars (map first (second code)) ) |
(exprs (map second (second code)) ) |
(body (rest (rest code)) ) |
) |
; recursively transform |
(trans vars (map transform-let-to-apply exprs) (map transform-let-to-apply body)) |
) |
|
|
(if (list? code) |
; recursively transform expressions |
(map transform-let-to-apply code) |
; leaf expression, leave alone |
code) |
)) |
|
|
(define c1 |
'(if #t |
"c1-true" |
"c1-false" |
)) |
|
(define c2 |
'(if #f |
(if (and #t (if #f #t #f)) |
"c11" |
"c12" |
) |
"c21" |
)) |
|
(define c3 |
'(let ( (x 1) (y (+ 2 3)) (z (if #t 42 -1)) ) (println "let-1") (+ x y z) ) |
) |
|
(define code-1 |
'(let ( (x 1) (y 2) ) |
;(println (list "got" x y)) |
(let ( (z (+ x y)) ) |
;(println (list "z" z)) |
(if (< x y) |
(list "x smaller") |
(list "y smaller") |
) |
) |
)) |
|
; why this does not work is explained in http://docs.racket-lang.org/guide/eval.html |
; the definitions window does not have a namespace by default. |
(define ns (make-base-namespace)) |
|
(define (test1) |
(println c1) |
(println (eval c1 ns)) |
(println (transform-if-to-cond c1)) |
(println (eval (transform-if-to-cond c1) ns)) |
(newline) |
(println c2) |
(println (eval c2 ns)) |
(println (transform-if-to-cond c2)) |
(println (eval (transform-if-to-cond c2) ns)) |
) |
|
;(test1) |
|
; our own println |
; displayln will drop the " " around strings, which makes it difficult to |
; read the transformed code. So we use println instead. |
(define (println s) (print s) (newline)) |
|
(define (test2a) |
(println c3) |
(println (eval c3 ns)) |
(println (transform-let-to-apply c3)) |
(println (eval (transform-let-to-apply c3) ns)) |
(newline) |
(println code-1) |
(println (eval code-1 ns)) |
(println (transform-let-to-apply code-1)) |
(println (eval (transform-let-to-apply code-1) ns)) |
|
) |
|
(define (test-1) |
(newline) |
(println code-1) |
(println (eval code-1)) |
(newline) |
|
; test the if transform |
(define t1 (transform-if-to-cond code-1)) |
(println t1) |
(println (eval t1)) |
(newline) |
|
; test the let transform |
(define t2 (transform-let-to-apply code-1)) |
(println t2) |
(println (eval t2)) |
(newline) |
|
; test the two different composition orders of the transforms |
(define t3 (transform-let-to-apply (transform-if-to-cond code-1))) |
(println t3) |
(println (eval t3)) |
(newline) |
(define t4 (transform-if-to-cond(transform-let-to-apply code-1))) |
(println t4) |
(println (eval t4)) |
) |
-- data types are defined by constructor patterns |
-- Eq equality can be derived from structural equality |
|
-- a binary tree consisting of leaves L or internal nodes N |
data BinTree = L | N1 BinTree | N BinTree BinTree deriving (Eq, Show) |
|
-- this function creates the full binary tree of size 2^(n+1) -1 |
makeBinTree 0 = L |
makeBinTree n = N (makeBinTree (n-1)) (makeBinTree (n-1)) |
|
-- this function computes the size of a binary tree |
size L = 1 |
size (N1 t1) = 1 + size t1 |
size (N t1 t2) = 1 + (size t1) + (size t2) |
|
-- this function computes the depth of a binary tree |
depth L = 1 |
depth (N1 t1) = 1 + (depth t1) |
depth (N t1 t2) = 1 + max (depth t1) (depth t2) |
|
-- makeABinTree creates a binary tree of size s of minimum depth |
-- in mixed prefix/infix form: (quot (s-1) 2) + (rem (s-1) 2) |
-- in pure prefix form: ( (+) (quot (s-1) 2) (rem (s-1) 2)) |
|
makeABinTree :: Integral a => a -> BinTree |
makeABinTree s |
| s <= 1 = L |
| s == 2 = N1 L |
| otherwise = |
N (makeABinTree (sizeLeft s)) (makeABinTree (sizeRight s)) |
where |
sizeLeft :: Integral a => a -> a |
sizeRight :: Integral a => a -> a |
|
sizeLeft s = max 1 ( (s-1) `quot` 2 + (s-1) `rem` 2) |
sizeRight s = max 1 ((s-1) `quot` 2) |
|
|
if
expressions. It negates the condition and exchanges the true and false parts, thus preserving the logic but rewriting the expression. #lang racket |
|
(define (modify-if trans code) |
; locates every occurrence of an if expression in code and |
; applies the transform function to it. |
|
(if (and (list? code) (not (null? code)) (eq? 'if (first code))) |
; found an (if ...) expr, transform the parts |
(trans (second code) (third code) (fourth code) ) |
|
; not an if, so if a ( ) then transform each of the parts |
; otherwise leave alone |
(if (list? code) |
(map (lambda (c) (modify-if trans c)) code) |
code) |
)) |
|
|
(define c1 |
'(if true |
"c1-true" |
"c1-false" |
)) |
|
(define c2 |
'(if false |
(if (and true (if false true false)) |
"c11" |
"c12" |
) |
"c21" |
)) |
|
; transform (if c p q) into (if (not c) q p) |
(define (trans-1 c p q) |
(list 'if (list 'not c) q p) |
) |
|
(define (test1) |
(displayln (eval c1)) |
(displayln (modify-if trans-1 c1)) |
(displayln (eval (modify-if trans-1 c1))) |
(newline) |
(displayln (modify-if trans-1 c2)) |
) |
|
; (test1) |
#lang racket |
|
(define (modify-if trans code) |
; locates every occurrence of an if expression in code and |
; applies the transform function to it. |
|
; remember the trans function |
(define (mf c) (modify-if trans c)) |
|
(if (and (list? code) (not (null? code)) (eq? 'if (first code))) |
|
; recursivley transform the parts of the if before transforming the if |
(apply trans (map mf (rest code))) |
; Note above is better than |
; (trans (mf (second code)) (mf (third code)) (mf (fourth code))) |
|
(if (list? code) |
; recursively transform expressions |
(map mf code) |
; leaf expression, leave alone |
code) |
)) |
|
|
(define c1 |
'(if true |
"c1-true" |
"c1-false" |
)) |
|
(define c2 |
'(if false |
(if (and true (if false true false)) |
"c11" |
"c12" |
) |
"c21" |
)) |
|
(define (wrap-not x) |
(list 'not x) ) |
|
; transform (if c p q) into (if (not c) q p) |
(define (trans-1 c p q) |
(list 'if (wrap-not c) q p) |
) |
|
(define (test1) |
(displayln c1) |
;(displayln (eval c1)) |
(displayln (modify-if trans-1 c1)) |
(displayln (eval (modify-if trans-1 c1))) |
(newline) |
(displayln c2) |
(displayln (modify-if trans-1 c2)) |
) |
|
; (test1) |
if
into a cond
. #lang racket |
|
; a e-tree represents the kinds of program structures that we want to |
; manipulate. Defined recursively: |
|
; atom := a single primative unstructured element, like a number 42 |
; or symbol 'x |
|
; e-tree := atom | ( e-tree * ) |
; that is, an e-tree is an atom, or a list of 0 or more e-trees |
|
; a path in an e-tree is a list of indexes that navigate to a |
; sub-component of the e-tree. Navigation via a path is defined |
; by the following function, modelled after list-ref |
|
(define (e-tree-ref e-tree path) |
(if (null? path) e-tree |
(e-tree-ref (list-ref e-tree (first path)) (rest path))) |
) |
|
; examples: |
(define (test-e-tree-ref) |
(define t1 '(a (b c) (d (e f) (g h (i j))))) |
|
(map displayln (list |
t1 |
" " |
(e-tree-ref t1 '()) |
(e-tree-ref t1 '(0)) |
(e-tree-ref t1 '(2)) |
(e-tree-ref t1 '(2 1 1)) |
(e-tree-ref t1 '(2 2 2 1)) |
|
)) |
(void) |
) |
|
; given an e-tree and a path, we would like to edit the sub e-tree |
; at the endpoint of the path. |
|
; first we need a primative for editing an element a list |
|
; given editing function edit-fn, list l, and position n inside l, |
; return a list |
; which is l with (list-ref l n) replaced with (fn (list-ref l n)) |
|
(define (list-edit edit-fn l n) |
(let-values |
( [ (a b) (split-at l n) ] ) ; note capture a 2 value result |
|
(if (null? b) |
(error (format "edit-list n of ~a >= length l of ~a" n l)) |
(append a (list (edit-fn (first b))) (rest b)) |
) |
) |
) |
|
; given tree e-tree, and path edit-path, return an e-tree |
; which is the original with the subtree t reached by edit-path |
; repaced by (edit-fn t) |
|
(define (e-tree-edit edit-fn e-tree edit-path) |
(cond |
; empty path edits entire e-tree |
[ (null? edit-path) (edit-fn e-tree) ] |
|
; for longer path, replace the (first edit-path) position in e-tree with the |
; result of recursing down the sub e-tree at that position using the rest |
; of the edit path |
|
[ else (list-edit |
(lambda (sub-e-tree) (e-tree-edit edit-fn sub-e-tree (rest edit-path))) |
e-tree |
(first edit-path)) |
] |
)) |
|
(define (test-e-tree-edit) |
(define t1 '(a (b c) (d (e f) (g h (i j))))) |
|
(map displayln (list |
t1 |
" " |
(e-tree-edit (lambda (s) (rest s)) t1 '()) |
(e-tree-edit (lambda (s) 'f1) t1 '(0)) |
(e-tree-edit (lambda (s) (list (second s) (first s))) t1 '(1)) |
(e-tree-edit (lambda (s) (list 'f2 s) ) t1 '(2 1 1)) |
(e-tree-edit (lambda (s) (list 'lambda (list (e-tree-ref s '(1))) s) ) t1 '(2 2 2)) |
|
)) |
(void) |
) |
|
; perform the same edit at multiple points in the tree. You need |
; to be careful if the paths intersect in the sense that the endpoint |
; of one path is in a sub e-tree below the endpoint of another path. |
|
(define (e-tree-edit-many edit-fn in-tree edit-paths) |
(foldl (lambda (path cur-e-tree) (e-tree-edit edit-fn cur-e-tree path)) |
in-tree edit-paths) |
) |
|
(define (test-e-tree-edit-many) |
(define t1 '(a (b c) (d (e f) (g h (i j))))) |
|
(map displayln (list |
t1 |
" " |
(e-tree-edit-many (lambda (s) 'f1) t1 '( (0) (1 1))) |
(e-tree-edit-many (lambda (s) (list 'f2 s) ) t1 '( (2 1 1) (2 2 2)) ) |
|
)) |
(void) |
) |
|
; find paths to all instances of term in expression expr |
; A path is a list of 0-origin indexes into the expression tree |
; indicating how to navigate down to the root to locate the term |
|
(define (e-tree-find-paths target-e-tree e-tree [paths '()] [rootpath '()] ) |
(cond |
; are we at a matching term? |
[ (equal? target-e-tree e-tree) (append paths (list rootpath) ) ] |
|
; ok, no match, no recursion since e-tree empty |
[ (null? e-tree) paths ] |
|
; recurse into sub e-trees accumulating paths to matches |
[ (list? e-tree) |
(second |
(foldl |
; (e s) is current sub e-tree e, prior state s |
; s state is (n p) where |
; n is position of e in containing e-tree, |
; p collection of paths found so far |
(lambda (e s) |
(let ( |
; pull apart state |
(n (first s)) |
(p (second s)) |
) |
; new state - next position, add newly found paths |
(list |
(add1 n) |
(e-tree-find-paths target-e-tree e p (append rootpath (list n))) |
) |
) |
) |
(list 0 paths) e-tree |
) |
) ] |
|
[ else paths ] |
) |
) |
|
(define (test-e-tree-find-paths) |
(define t1 '(a (b a) (b (a f) (f h (b a))))) |
|
(map displayln (list |
t1 |
" " |
(e-tree-find-paths 'a t1) |
(e-tree-find-paths 'b t1) |
(e-tree-find-paths '(b a) t1) |
|
|
(e-tree-edit-many (lambda (s) 'delta) t1 (e-tree-find-paths 'a t1)) |
(e-tree-edit-many (lambda (s) 'delta) t1 (e-tree-find-paths '(b a) t1) ) |
)) |
(void) |
) |
|
; NOTE: write a version of e-tree-find-paths that has a test predicate |
; other than equal? |
|
; Program transformations |
|
; Here is an attempt to extract out the continuation for a term |
; inside a e-tree. |
|
; assuming that e-tree contains non special forms, then extract the |
; continuation context about the term at point path. |
|
|
(define (extract-continuation-1 e-tree path) |
(cond |
; if the path has length 1, we are at a term ( ... ) |
|
; in first position of list, it is a fn call |
[ (equal? path '(0)) '(lambda (x) x) ] |
|
; in other positions it is a term to evaluate then pass up to |
; enclosing expressions |
[ (null? (rest path)) |
(let ( [ var (gensym 'v- ) ] ) |
(list 'lambda (list var) (list-edit (lambda (s) var) e-tree (first path)) ) |
)] |
|
; the path descends, so we need to capture the context around and |
; descend further |
|
[ else |
(let ( [ var (gensym 'v- ) ] ) |
(list 'lambda (list var) (e-tree-edit (lambda (s) var) e-tree path)) |
)] |
|
|
)) |
|
(define (f-test x y) |
(if (null? x) y (cons (first x) (f-test (rest x) (cons (first x) y))) ) |
) |
|
; run this test by doing |
; (eval (test-convert)) |
; (test-cc '(a b) '(1 2)) |
; you should get: '(a b b a 1 2) |
; |
(define (test-convert) |
(define e1 ' |
(define (f x y) |
(if (null? x) y (cons (first x) (f (rest x) (cons (first x) y))) ) |
)) |
|
(eval (convert-to-cc-style e1 'test-cc)) |
|
) |
|
; converts a cannonical recursive definition into continuation- |
; passing style. |
; the function MUST have the form |
; (define (f ... ) |
; (if (basecase? ...) base-expression recursion-expression) ) |
; |
(define (convert-to-cc-style defn new-fn-name) |
|
(or (eq? 'if (e-tree-ref defn '(2 0))) |
(error (format "not a if-style definition ~a" defn))) |
|
; get the parts of the definition |
(let ( |
[ args (e-tree-ref defn '(1)) ] |
[ fn-name (e-tree-ref defn '(1 0)) ] |
[ base-test (e-tree-ref defn '(2 1)) ] |
[ base-case (e-tree-ref defn '(2 2)) ] |
[ recursion (e-tree-ref defn '(2 3)) ] |
) |
|
|
; locate the recursive call in the recursion |
(define p1 (e-tree-find-paths fn-name recursion)) |
|
; make sure only one call |
(or (= 1 (length p1)) |
(error (format "not a single recursion in ~a" defn) ) ) |
|
; move up to expression that needs result of recursive call |
; and make that the continuation |
(define cc1 (list-edit |
(lambda (s) (list 'cc s)) |
(extract-continuation-1 recursion (drop-right (first p1) 1)) |
2)) |
(displayln cc1) |
|
; insert continuation into the original recursion |
(define new-rec (append (e-tree-ref recursion (drop-right (first p1) 1)) (list cc1))) |
|
(displayln new-rec) |
|
; create the default continuation |
|
; rewrite defintion to be in continuation passing style |
(define new-def |
(list 'define (append (second defn) '( (cc (lambda (x) x)))) |
(list 'if base-test (list 'cc base-case) new-rec) ) |
) |
|
|
(displayln new-def) |
|
; rename the original function to the new name |
(define final-def |
(e-tree-edit-many (lambda (s) new-fn-name) new-def (e-tree-find-paths fn-name new-def)) ) |
|
(displayln final-def) |
|
; then eval the final result to create the new function. |
|
final-def |
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)) |
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8. Week 6 - Feb 10 CMPUT 325 Schedule / Version 2.31 2014-04-04 |