Mercurial > urweb
changeset 408:de3b03e22447
ListShop prose
| author | Adam Chlipala <adamc@hcoop.net> |
|---|---|
| date | Tue, 21 Oct 2008 17:44:03 -0400 |
| parents | 345fcf91c806 |
| children | 81d9f42bb641 |
| files | demo/prose |
| diffstat | 1 files changed, 7 insertions(+), 1 deletions(-) [+] |
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--- a/demo/prose Tue Oct 21 17:34:20 2008 -0400 +++ b/demo/prose Tue Oct 21 17:44:03 2008 -0400 @@ -50,4 +50,10 @@ listShop.urp -<p>This is my other favorite.</p> +<p>This example shows off algebraic datatypes, parametric polymorphism, and functors.</p> + +<p>The <tt>List</tt> module defines a list datatype, much in the style of SML, but with type parameters written more in Haskell style. The types of <tt>List.length</tt> and <tt>List.rev</tt> indicate that they are polymorphic. Types like <tt>t ::: Type -> ...</tt> indicate polymorphism, with the triple colon denoting that the value of this type parameter should be <i>inferred</i> at uses. A double colon would mean that the type argument must be provided explicitly at uses. In contrast to ML and Haskell, all polymorphism must be <i>declared</i> explicitly in Ur, while instantiations may be inferred at uses.</p> + +<p>The <tt>ListFun</tt> module defines a functor for building list editing sub-applications. An argument to the functor <tt>Make</tt> must give the type to be stored in the lists, along with marshaling and unmarshaling functions. In return, the functor returns an entry point function.</p> + +<p>The <tt>ListShop</tt> modules ties everything together by instantiating <tt>ListFun.Make</tt> with structures for integers and strings. <tt>show</tt> and <tt>read</tt> can be used for marshaling and unmarshaling in both cases because they are type-class-generic.</p>