Differences between version 16 and predecessor to the previous major change of ML.
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| Newer page: | version 16 | Last edited on Monday, February 24, 2003 3:22:18 pm | by GlynWebster | Revert |
| Older page: | version 15 | Last edited on Thursday, February 20, 2003 5:04:31 pm | by GlynWebster | Revert |
@@ -43,34 +43,9 @@
[2]
!!!Polymorphism
-This is
what prevents ML's strong type checking from being a pain in the bum. You don't have to define the type of everything ''exactly'', you can leave some types, or parts of some types unspecified. For example this is a type for binary trees:
-
- # __type__ 'a tree = Leaf __of__ 'a | Node __of__ 'a tree * 'a tree ;;
- ''type 'a tree = Leaf of 'a | Node of 'a tree * 'a tree''
-
-(That's me typing at the Ocaml interpreter prompt and Ocaml responding.) The type __tree__ is polymorphic: __'a__ is a place holder for any type. For example
you might want a __string tree__:
-
- # __let__ my_tree = Node (Node (Leaf "Cow", Leaf "Pig"), Leaf "Cat") ;;
- ''val my_tree : string tree = Node (Node (Leaf "Cow", Leaf "Pig"), Leaf "Cat")''
-
-The Ocaml interpreter was in fact clever enough to work that was a string tree by itself. From my tree type I can make __int trees__, __char trees__, __string tree trees__, whatever. These would all be incompatible types, because __ints__, __chars__ and __strings__ are incompatible. However, functions I
write for __'
a tree__s can be used on any
type of tree:
-
- # __let rec__ count_leaves (t : 'a tree) : int =
- __match__ t __with__
- | Leaf _
-> 1
- | Node (left,right) -> count_leaves left + count_leaves right ;;
- ''val count_leaves : 'a tree -> int = <fun>''
-
- # count_leaves my_tree ;;
- ''- : int = 3''
-
- # count_leaves (Node (Leaf 1, Leaf 2)) ;;
- ''- : int = 2''
-
-Polymorphism is something ML has in common with [Haskell]
.
-
+PolymorphicTypes are
what prevents ML's strong type checking from being a pain in the bum. You don't have to define the type of everything ''exactly'', you can leave some types, or parts of some types unspecified. This allowed
you write a great variety of
type-safe generic functions
.
[3]
!!!Higher-order Functions
