[Haskell-cafe] Fwd: [Haskell-beginners] Monad instances and type synonyms

[Chris Wong]

On Sun, Apr 14, 2013 at 5:10 PM, Christopher Howard
<christopher.howard at frigidcode.com> wrote:
> type Adjustment a = SaleVariables -> a
>
> [...]
>
> instance Monad Adjustment where
>
>   (>>=) = ...
>   return = ...

Essentially, you can't partially apply type synonyms. I don't recall
the exact reasoning, but if this sort of thing was allowed it would
probably poke funny holes in the type system.

Also, Control.Monad.Instances already supplies a Monad instance for
functions (r -> a). So even if that did pass, you'd bump into
overlapping instances anyway.

Chris

> --------
> If I try this, I get
>
> code:
> --------
> Type synonym `Adjustment' should have 1 argument, but has been given none
> In the instance declaration for `Monad Adjustment'
> --------
>
> But if I give an argument, then it doesn't compile either (it becomes a
> "*" kind). And I didn't want to make the type with a regular "data"
> declaration either, because then I have to give it a constructor, which
> doesn't fit with what I want the type to do.
>
> --
> frigidcode.com
>
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--
Chris Wong, fixpoint conjurer
  e: lambda.fairy at gmail.com
  w: http://lfairy.github.io/