David, hello again! Interesting is that class with one parameter is fine: class Repr a where type ReprAs a reprx :: a -> ReprAs a instance Repr Int where type ReprAs Int = Int reprx n = n main = do let n = 5 :: Int print $ (reprx n) but when I added 2nd param, I get ambogouse error again: class Repr a b where type ReprAs a b repr :: a -> ReprAs a b instance Repr Int AsTitle where type ReprAs Int AsTitle = String repr n = show n main = do let n = 5 :: Int print $ (repr n::ReprAs Int AsTitle) which looks the same as with type-alias: GHC says: 30 12 error error: • Couldn't match type ‘ReprAs Int b0’ with ‘String’ Expected type: ReprAs Int AsTitle Actual type: ReprAs Int b0 The type variable ‘b0’ is ambiguous • In the second argument of ‘($)’, namely ‘(repr n :: ReprAs Int AsTitle)’ In a stmt of a 'do' block: print $ (repr n :: ReprAs Int AsTitle) In the expression: do { let n = ...; print $ (repr n :: ReprAs Int AsTitle) } (intero) looks that `type ReprAs Int AsTitle` is treating type-alias and GHC can not match resulting `String` with `ReprAs Int AsTitle`. As I understand, the root of the problem is that class parameters are not the same as in results' parameters (contraposition types) - they turn out to be free/unbound (execuse my English). For example, to avoid this problem in the class I added extension: {-# LANGUAGE AllowAmbiguousTypes #-} I don't know is a way in Haskell to say to reuse already bound class parameters in "methods" bodies... I found this: https://stackoverflow.com/questions/4174187/reading-and-representing-input-w... Another variant which is compiling (w/ func-deps): {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE MultiParamTypeClasses #-} data AsShow data AsTitle type ReprAs a = String class Repr a b | a -> b where repr :: a -> ReprAs b instance Repr Int AsTitle where repr n = "a number '" ++ show n ++ "'" -- instance Repr Int AsShow where -- repr n = "a number '" ++ show n ++ "'" main = do let n = 5 :: Int print $ (repr n::ReprAs AsTitle) print $ (repr n::ReprAs AsShow) but due to `a -> b` it's impossible to instantiate another `Repr Int`! So, I think soultion is in: - func deps - or type families/associative types But unfortunately my knowledge of Haskell is limited to find it. I see only that I need to add another param to func dep of class - to "extend" dependency which allows to determine result type not only on one input argument's type... Thanks David! === Best regards, Paul

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 wrote:

...

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

---

Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

Hello, Sylvain. Your solution assumes that I need to pass `@_ @AsTitle` anywhere where I call `repr`? So, instead of `repr n::AsTitle` or `repr n::Something AsTitle` (depends on implementation) I'll write `repr @_ @AsTitle n`, right?

You only add a constraint as follows: repr n :: forall b. (Repr b ~ Repr AsTitle, ReprC Int b) => Repr b

Yes... So, `b` is unbound/free type param. But I can bind it with func. deps, yes? === Best regards, Paul

But as we have seen, it doesn't imply (b ~ AsTitle) and b remains ambiguous.

A solution is to fix b explicitly with a type application:

{-# LANGUAGE TypeApplications #-} ... repr @_ @AsTitle n

Best regards, Sylvain

On 27/09/2017 17:50, Baa wrote:

...

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

_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

You could also avoid all this ambiguity/type application stuff with: data AsTitle = AsTitle class Repr a b where   repr :: a -> b -> Repr b instance Repr Int AsTitle where   repr n _ = ... ... print (repr n AsTitle)

Yes, "moving" of type from the right-side (result) to the left-side (arguments) solves the problem, sure.

(Repr b can be an associated type if you want to support different representations)

...

...

You only add a constraint as follows: repr n :: forall b. (Repr b ~ Repr AsTitle, ReprC Int b) => Repr b Yes... So, `b` is unbound/free type param. But I can bind it with func. deps, yes?

How? If you you bind it with a functional dependency on "a", you can only have a single "b" for each "a".

I don't know... I wrote to David my idea - additional param on the left-side of func. dependency, but how/what param? === Best regards, Paul