1 #! A lambda expression manipulator, by Matthew Willis
2 USING: lazy-lists strings arrays hashtables
3 sequences namespaces words parser kernel ;
7 : dip swap slip ; inline
10 TUPLE: lambda-node self expr name ;
11 TUPLE: apply-node func arg ;
12 TUPLE: var-node name ; #! var is either a var, name, or pointer to a lambda-node
13 TUPLE: beta-node expr lambdas ; #! a namespace node
14 TUPLE: alien-node word ;
16 M: lambda-node equal? eq? ;
19 M: lambda-node bind-var ( binding lambda -- )
20 lambda-node-expr bind-var ;
22 M: apply-node bind-var ( binding apply -- )
23 [ apply-node-func bind-var ] 2keep apply-node-arg bind-var ;
25 M: var-node bind-var ( binding var-node -- )
26 2dup var-node-name swap lambda-node-name =
27 [ set-var-node-name ] [ 2drop ] if ;
29 M: alien-node bind-var ( binding alien -- ) 2drop ;
31 C: lambda-node ( expr var lambda -- lambda )
32 swapd [ set-lambda-node-name ] keep
33 [ set-lambda-node-expr ] 2keep
34 dup [ set-lambda-node-self ] keep
35 [ swap bind-var ] keep ;
38 #! push the beta further down the syntax tree
39 #! this is how lambda achieves lazy beta reduction and efficient cloning.
40 #! everything outside of the beta must have been cloned.
41 M: lambda-node beta-push ( beta lambda -- lambda )
42 clone dup lambda-node-expr pick set-beta-node-expr
43 [ set-lambda-node-expr ] keep ;
45 M: apply-node beta-push ( beta apply -- apply )
46 #! push the beta into each branch, cloning the beta
48 pick apply-node-func swap [ set-beta-node-expr ] keep swap
49 rot apply-node-arg swap [ set-beta-node-expr ] keep
52 M: var-node beta-push ( beta var -- expr )
53 #! substitute the variable with the appropriate entry from the
55 tuck var-node-name swap beta-node-lambdas hash dup
58 M: beta-node beta-push ( beta inner-beta -- beta )
59 #! combines the namespaces of two betas
60 dup beta-node-lambdas rot beta-node-lambdas hash-union
61 swap [ set-beta-node-lambdas ] keep ;
63 M: alien-node beta-push ( beta alien -- alien ) nip ;
65 : beta-reduce ( apply -- beta )
66 #! construct a beta-node which carries the namespace of the lambda
67 dup apply-node-arg swap apply-node-func dup lambda-node-expr -rot
68 lambda-node-self H{ } clone [ set-hash ] keep <beta-node> ;
71 : left-reduce ( apply -- apply/f )
72 #! we are at an application node -- evaluate the function
73 dup apply-node-func evaluate dup
74 [ swap [ set-apply-node-func ] keep ]
77 : alien-reduce ( apply -- node/f )
78 #! we have come to an alien application, which requires us to
79 #! fully normalize the argument before proceeding
80 dup apply-node-arg evaluate dup
81 [ swap [ set-apply-node-arg ] keep ]
82 [ #! right side is normalized, we are ready to do the alien application
83 drop dup apply-node-arg swap apply-node-func
84 alien-node-word "lambda" lookup execute
89 #! beta-reduction, beta-pushing, and name replacing.
90 : normalize ( expr -- expr )
91 dup evaluate [ nip normalize ] when* ;
93 M: lambda-node evaluate ( lambda -- node/f ) drop f ;
95 M: apply-node evaluate ( apply -- node )
96 dup apply-node-func lambda-node?
99 dup apply-node-func alien-node?
104 M: var-node evaluate ( var -- node/f )
105 var-node-name lambda-names get hash ;
107 M: beta-node evaluate ( beta -- node/f )
108 dup beta-node-expr beta-push ;
110 M: alien-node evaluate ( alien -- node/f ) drop f ;
113 M: lambda-node expr>string ( lambda-node -- string )
115 dup "(" , lambda-node-name , ". " ,
116 lambda-node-expr expr>string , ")" ,
119 M: apply-node expr>string ( apply-node -- string )
121 dup "(" , apply-node-func expr>string , " " ,
122 apply-node-arg expr>string , ")" ,
125 M: var-node expr>string ( variable-node -- string )
126 var-node-name dup string? [ lambda-node-name ] unless ;
128 M: alien-node expr>string ( alien-node -- string )
129 [ "[" , alien-node-word , "]" , ] { } make concat ;
131 M: beta-node expr>string ( beta -- string )
132 [ "beta<" , beta-node-expr expr>string , ">" , ] { } make concat ;