-#Scheme and OCaml#
-
-* You can [try Scheme in your web browser](http://tryscheme.sourceforge.net/). This is useful if you don't have Racket or another Scheme implementation installed---but don't expect it to have all the bells and whistles of a mature implementation!
-
-* **Type Variants and Pattern Matching** If you want to reproduce this kind of OCaml code:
-
- # type lambda_expression = Var of char | Lam of char * lambda_expression | App of lambda_expression * lambda_expression;;
-
- # let rec free_vars (expr : lambda_expression) : char list =
- match expr with
- | Var label -> [label]
- | Lam (label, body) -> remove label (free_vars body)
- | App (left, right) -> merge (free_vars left) (free_vars right);;
-
- # free_vars (Lam ('x', (App (Var 'x', Var 'y'))));;
- - : char list = ['y']
-
- in Scheme, you have two choices. First, the quick hack:
-
- ; we use the symbols 'var and 'lam as tags, and assume
- ; that an expression will always be a pair of one of these forms:
- ; (cons 'var symbol)
- ; (cons (cons 'lam symbol) expression)
- ; (cons expression expression)
-
- (define (free-vars expr)
- (cond
- [(eq? (car expr) 'var) (list (cdr expr))]
- [(and? (pair? (car expr)) (eq? (car (car expr)) 'lam))
- (remove (cdr (car expr)) (free-vars (cdr expr)))]
- [else (merge (free-vars (car expr)) (free-vars (cdr expr)))]))
-
- Second, you can create real datatypes and pattern-match on them. There are several tools for doing this. I'll describe the `define-datatype` and `cases` forms developed for the book *Essentials of Programming Languages* (EoPL) by Friedman and Wand.
-
- (Alternatives include [the `struct` form in Racket](http://docs.racket-lang.org/guide/define-struct.html). Also `define-record-type` from srfi-9 and srfi-57; see also [the r6rs libs](http://docs.racket-lang.org/r6rs-lib-std/r6rs-lib-Z-H-7.html).)