The reason is because 'type' is a type alias that changes nothing code
wise, it is merely a visual clue to the reader of code that these two
types are the same. You could replace 'Repr AsTitle' with 'String'
and you would get the exact same error.
That said I do think you can do what you want with type families, but
I'm not having luck giving you a complete solution at this time. This
would be a good question to ask on stackoverflow if no one here gives
a satisfactory answer. Here is the code I had that doesn't quite
work, although I'm not sure why. Maybe you can figure it out. I
would love to know.
{-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}
data AsShow
data AsTitle
class Repr a b where
type ReprC a b :: *
repr :: a -> ReprC a b
instance Repr Int AsTitle where
type ReprC Int AsTitle = String
repr n = show n
main = do
let n = 5 :: Int
print $ (repr n)
On Wed, Sep 27, 2017 at 11:50 AM, Baa
Hello, List!
The further - the more interesting... For example, I want own `Show` analogue, which will be parameterized with type meaning "context"/"reason". So, to represent title of the value I will call:
repr x::TitleRepr
or simple `Show`:
repr x::ShowRepr
or
repr x::Repr AsShow
etc. I try:
{-# LANGUAGE AllowAmbiguousTypes #-} {-# LANGUAGE MultiParamTypeClasses #-}
data AsShow data AsTitle
type Repr a = String
class ReprC a b where repr :: a -> Repr b
instance ReprC Int AsTitle where repr n = "a number " ++ show n
main = do let n = 5 :: Int print $ (repr n :: Repr AsTitle)
and sure I get error:
• Ambiguous type variable ‘b0’ arising from a use of ‘repr’ prevents the constraint ‘(ReprC Int b0)’ from being solved. Probable fix: use a type annotation to specify what ‘b0’ should be. These potential instance exist: instance ReprC Int AsTitle -- Defined at .../.stack-work/intero/intero31144sPV.hs:12:10 • In the second argument of ‘($)’, namely ‘(repr n :: Repr AsTitle)’ In a stmt of a 'do' block: print $ (repr n :: Repr AsTitle) In the expression: do { let n = ...; print $ (repr n :: Repr AsTitle) } (intero)
Sure, I can use as `Repr` not type-alias but `newtype`. But in this case I will need additional call (show/runRepr/coerce/etc.):
coerce $ (repr x::AsTitle)
So, what is the reason that GHCI is see that `repr n` is `::Repr AsTitle` (AsTitle!!) and says me that `b0` is ambigous?! It should know what concreate `repr` I mean! :) If I use `newtype` - no problem, so I suppose problem is in the type alias. It's not sterling type for such goal, right?
Another question is: how to accomplish such goal, i.e. without to make additional call (coerce/show/runRepr/etc) when `repr` will return `String`, wrapped in newtype?
PS. Execuse such silly and training example. Actually, I planned to made such class and to use it instead of `Show`. Sure, it can be splitted to several classed (my current state) but.. example is to learn Haskell and to understand my errors...
=== Best regards, Paul _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners