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, see <http://docs.racket-lang.org/guide/define-struct.html>. Also `define-record-type` from srfi-9 and srfi-57; see also <http://docs.racket-lang.org/r6rs-lib-std/r6rs-lib-Z-H-7.html>.)
+ (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:
(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 <http://docs.racket-lang.org/reference/cont.html?q=shift&q=do#%28part._.Classical_.Control_.Operators%29>.
+* 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):
There is also a library for using *undelimited* continuations in OCaml, but it's shakier than Oleg's delimited continuation library.
-We won't say any more about translating to and from Scheme.
+There are some more hints about Scheme [here](/assignment8/) and [here](/week1/). We won't say any more here.
+
#Haskell and OCaml#
type person = name * address;;
type 'a personal_data = PD of 'a;;
+* When a type only has a single variant, as with PersonalData, Haskell programmers will often use the same name for both the type and the value constructor, like this:
+
+ data PersonalData a = PersonalData a
+
+ The interpreter can always tell from the context when you're using the type name and when you're using the value constructor.
+
* The type constructors discussed above took simple types as arguments. In Haskell, types are also allowed to take *type constructors* as arguments:
data BarType t = Bint (t Integer) | Bstring (t string)
which can be translated straightforwardly into OCaml.
+ For more details, see:
+
+ * [Haskell Wikibook on do-notation](http://en.wikibooks.org/wiki/Haskell/do_Notation)
+ * [Do-notation considered harmful](http://www.haskell.org/haskellwiki/Do_notation_considered_harmful)
+
* If you like the Haskell do-notation, there's [a library](http://www.cas.mcmaster.ca/~carette/pa_monad/) you can compile and install to let you use something similar in OCaml.
* In order to do any printing, Haskell has to use a special `IO` monad. So programs will look like this: