[Haskell-cafe] Re: Has anyone looked into adding subtyping to Haskell?

[Tomasz Zielonka]

On Thu, May 31, 2007 at 06:16:20PM +0100, Jon Harrop wrote:
> > I can't think of a lightweight way to encode overlapping enumerations in
> > Haskell. 
> 
> I'd like to know if this is possible in Haskell.

Maybe this way using GADTs and typeclasses? I haven't used such code in
practice - there may be some hidden traps.

    {-# OPTIONS -fglasgow-exts #-}

    module Enum where

    data Height
    data Size

    class HasMEDIUM t
    instance HasMEDIUM Height
    instance HasMEDIUM Size

    data ENUM t where
        LOW     :: ENUM Height
        MEDIUM  :: HasMEDIUM t => ENUM t
        HIGH    :: ENUM Height
        SMALL   :: ENUM Size
        BIG     :: ENUM Size

Example use:

    *Enum> :t [MEDIUM, LOW]
    [MEDIUM, LOW] :: [ENUM Height]
    *Enum> :t [MEDIUM, SMALL]
    [MEDIUM, SMALL] :: [ENUM Size]
    *Enum> :t [MEDIUM, SMALL, LOW]
    <interactive>:1:16:
        Couldn't match expected type `Size' against inferred type `Height'
          Expected type: ENUM Size
          Inferred type: ENUM Height
        In the expression: LOW
    
Best regards
Tomek