Mercurial > urweb
comparison demo/prose @ 408:de3b03e22447
ListShop prose
| author | Adam Chlipala <adamc@hcoop.net> |
|---|---|
| date | Tue, 21 Oct 2008 17:44:03 -0400 |
| parents | 345fcf91c806 |
| children | 81d9f42bb641 |
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| 407:345fcf91c806 | 408:de3b03e22447 |
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| 48 | 48 |
| 49 <p>In the implementation of <tt>handler</tt>, we see the notation <tt>{[...]}</tt>, which uses type classes to inject values of different types (<tt>string</tt> and <tt>bool</tt> in this case) into XML. It's probably worth stating explicitly that XML fragments <i>are not strings</i>, so that the type-checker will enforce that our final piece of XML is valid.</p> | 49 <p>In the implementation of <tt>handler</tt>, we see the notation <tt>{[...]}</tt>, which uses type classes to inject values of different types (<tt>string</tt> and <tt>bool</tt> in this case) into XML. It's probably worth stating explicitly that XML fragments <i>are not strings</i>, so that the type-checker will enforce that our final piece of XML is valid.</p> |
| 50 | 50 |
| 51 listShop.urp | 51 listShop.urp |
| 52 | 52 |
| 53 <p>This is my other favorite.</p> | 53 <p>This example shows off algebraic datatypes, parametric polymorphism, and functors.</p> |
| 54 | |
| 55 <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> | |
| 56 | |
| 57 <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> | |
| 58 | |
| 59 <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> |