[Haskell-cafe] Disjunctive patterns

[Serguey Zefirov]

2011/12/8 Asger Feldthaus <asger.feldthaus at gmail.com>:
> 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