#パターン

http://jijixi.azito.com/cgi-bin/diary/index.rb?date=20060622#p06

あれ？これexception使わなくても書けるような。

調べてみたら、#なんていうパターンがあった。

# type t = [`a of a | `b of b | ... `z of z]

みたいなtがあるときに、

# match x with
    | #t as t -> f t
    | `aa a -> g a;;

として、

# match x with
    (`a _ | `b _ | ... | `z _) as t -> f t
  | `aa a -> g a

の代わりにできるんだとか。

で、書き直してみた。

# type num = [`int of int | `float of float];;                                                                                           
type num = [ `float of float | `int of int ]

# let (+) x y = match (x,y) with                                                                                                         
       | (`int i, `int j) -> `int (i+j)                                                                                   
       | (`float f, `float g) -> `float (f +. g);;  
Warning P: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
(`float _, `int _)
val ( + ) :
  [< `float of float | `int of int ] ->
  [< `float of float | `int of int ] -> [> `float of float | `int of int ] =
  <fun>

# type str = [ num | `string of string];;                                                                                                
type str = [ `float of float | `int of int | `string of string ]

# let (+) x y = match (x,y) with                                                                                                        
      ((#num as a), (#num as b)) -> a + b                                                                                                
    | (`string s, `string t) -> `string(s^t);;                                                                                           
Warning P: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
(`string _, (`float _|`int _))
val ( + ) :
  [< `float of float | `int of int | `string of string ] ->
  [< `float of float | `int of int | `string of string ] ->
  [> `float of float | `int of int | `string of string ] = <fun>

めでたしめでたし。

ついでに、[> `float of ... ]みたいな型も消えました。