+## 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).)
+
+ Here is how the tools from EoPL work. You must begin your file either with `#lang eopl` or with the first two lines below:
+
+ #lang racket
+ (require eopl/eopl)
+
+ (define-datatype lambda-expression lambda-expression?
+ (var (label symbol?))
+ (lam (label symbol?) (body lambda-expression?))
+ (app (left lambda-expression?) (right lambda-expression?)))
+
+ (define (free-vars expr)
+ (cases lambda-expression expr
+ (var (label) (list label))
+ (lam (label body) (remove label (free-vars body)))
+ (app (left right) (remove-duplicates (append (free-vars left) (free-vars right))))))
+
+ (free-vars (lam 'x (app (var 'x) (var 'y))))
+ ; evaluates to '(y)
+
+* Scheme has excellent support for working with implicit or "first-class" **continuations**, using either `call/cc` or any of various delimited continuation operators. See [the Racket docs](http://docs.racket-lang.org/reference/cont.html?q=shift&q=do#%28part._.Classical_.Control_.Operators%29).
+
+ In Scheme you can use these forms by default (they're equivalent):
+
+ (call/cc (lambda (k) ...))
+ (let/cc k ...)
+
+ If your program declares `(require racket/control)`, you can also use:
+
+ (begin ... (reset ... (shift k ...) ...) ...)
+
+ (begin ... (prompt ... (control k ...) ...) ...)
+
+ (begin ... (prompt ... (abort value) ...) ...)
+
+ These last three forms are also available in OCaml, but to use them you'll need to compile and install Oleg Kiselyov's "delimcc" or "caml-shift" library (these names refer to the same library), which you can find [here](http://okmij.org/ftp/continuations/implementations.html#caml-shift). You'll already need to have OCaml installed. It also helps if you already have the findlib package installed, too, [as we discuss here](http://lambda.jimpryor.net/how_to_get_the_programming_languages_running_on_your_computer/). If you're not familiar with how to compile software on your computer, this might be beyond your reach for the time being.
+
+ But assuming you do manage to compile and install Oleg's library, here's how you'd use it in an OCaml session:
+
+ #require "delimcc";; (* loading Oleg's library this way requires the findlib package *)
+ (* if you don't have findlib, you'll need to start ocaml like
+ * this instead: ocaml -I /path/to/directory/containing/delimcc delimcc.cma
+ *)
+ open Delimcc;; (* this lets you say e.g. new_prompt instead of Delimcc.new_prompt *)
+ let p = new_prompt ();;
+ let prompt thunk = push_prompt p thunk;;
+ let foo =
+ ...
+ prompt (fun () ->
+ ...
+ shift p (fun k -> ...)
+ ...
+ (* or *)
+ control p (fun k -> ...)
+ ...
+ (* or *)
+ abort p value
+ ...
+ )
+ ...
+
+ There is also a library for using *undelimited* continuations in OCaml, but it's shakier than Oleg's delimited continuation library.
+
+<!--
+There are some more hints about Scheme [here](/assignment8/) and [here](/week1/). We won't say any more here.
+-->