On Mon, 27 Jul 1998, S.D.Mechveliani wrote:
> Obstacle 2:
> ------------
> Haskell rejects this `=> RightModule r r'
>
> How can we express the meaning
> ... RightModule m r where m is the identical constructor
> (m a = a)
> ?
> Scripting
> newtype Id a = Id a deriving(...)
>
> instance Ring r => RightModule Id r where
> cMul (Id r) r' = Id (mul r r')
> is an awkward way-out.
> Because from this point on, the programmer has to convert the data
> explicitly between r and Id r. This is not exactly what the
> application domain means.
Actually and strictly speaking, the `awkward' way out is here the correct
one, because it makes explicit the difference between the ring and the
canonical module over itself it determines.
The instance Ring r => RightMod Id r can be regarded as an expression of
the fact that Id : Rng -> Mod is a functor, (which can't be an identity
functor, because domain and codomain aren't the same!)
cheers,
-- m
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Mariano Suarez Alvarez
Departamento de Matematica
Universidad Nacional de Rosario
Pellegrini 250
2000 Rosario - Argentina
e-mail: [EMAIL PROTECTED]
El autor no responde de las molestias que puedan ocasionar sus escritos:
Aunque le pese
El lector tendra que darse siempre por satisfecho.
Nicanor Parra, `Poemas y antipoemas' (Advertencia al lector)
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