2011/12/8 Asger Feldthaus
Haskell doesn't seem to support disjunctive patterns, and I'm having a difficult time writing good Haskell code in situations that would otherwise call for that type of pattern.
Suppose for an example I have this data type:
data T = Foo Int | Bar Int | Baz
In OCaml I can write something like:
(* foo : T -> T -> int *) fun foo x y = match (x,y) with | (Foo a | Bar a, Foo b | Bar b) -> a + b
I solve that kind of problem by introducing operation enumerations. I can write expression definition like that: data Expr = Plus Int Int | Minus Int Int | Mul Int Int | Neg Int | Inv Int | Var String And then I will have exactly your problem. I prefer to write such definition like that: data Expr = Bin BinOp Int Int | Un UnOp Int | Var String data BinOp = Plus | Minus | Mul data UnOp = Neg | Inv And I have to write less code in all subsequent constructions and pattern matches. This is especially good when I used that method for an expression with result size: data Expr size where Bin :: BinOp xSize ySize resultSize -> Expr xSize -> Expr ySize -> Expr resultSize data BinOp a b r where Plus :: BinOp a a a Concatenate :: BinOp a b (Plus a b) Equal :: BinOp a a ONE