MPTCs and extended defaulting

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Tom Murphy

29 Aug 2016 29 Aug '16

4:23 p.m.

I've got a case in a library I'm working on where having -XExtendedDefaultRules with MPTCs would be very, very helpful. Is it possible? I.e., we can now write: ``` {-# LANGUAGE ExtendedDefaultRules #-} data AB = A | B Double deriving (Show) class Foo x where foo :: x -> AB instance Foo Double where foo = B main = print $ foo 5 ``` And -XExtendedDefaultRules makes sure we don't need to write "5 :: Double" If, though, I want 'Foo' to take another parameter (here, a :: [Symbol]), it falls apart: ``` {-# LANGUAGE DataKinds #-} {-# LANGUAGE ExtendedDefaultRules #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE MultiParamTypeClasses #-} import GHC.TypeLits data AB = A | B Double deriving (Show) class Foo x (a :: [Symbol]) where foo :: x -> AB instance Foo Double a where foo = B main = print $ foo 5 ``` Is there a reason MPTCs can't support ExtendedDefaultRules? Thanks! Tom

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Tom Murphy

22 Sep 22 Sep

7:39 p.m.

Bump -- is this feasible? It would be extremely helpful for an EDSL of mine. Tom On Tue, Aug 30, 2016 at 5:23 AM, Tom Murphy wrote:

...

I've got a case in a library I'm working on where having -XExtendedDefaultRules with MPTCs would be very, very helpful. Is it possible?

I.e., we can now write:

``` {-# LANGUAGE ExtendedDefaultRules #-}

data AB = A | B Double deriving (Show)

class Foo x where foo :: x -> AB

instance Foo Double where foo = B

main = print $ foo 5 ```

And -XExtendedDefaultRules makes sure we don't need to write "5 :: Double"

If, though, I want 'Foo' to take another parameter (here, a :: [Symbol]), it falls apart:

``` {-# LANGUAGE DataKinds #-} {-# LANGUAGE ExtendedDefaultRules #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE MultiParamTypeClasses #-}

import GHC.TypeLits

data AB = A | B Double deriving (Show)

class Foo x (a :: [Symbol]) where foo :: x -> AB

instance Foo Double a where foo = B

main = print $ foo 5 ```

Is there a reason MPTCs can't support ExtendedDefaultRules?

Thanks! Tom

Christopher Allen

7:56 p.m.

{-# LANGUAGE DataKinds #-} {-# LANGUAGE ExtendedDefaultRules #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TypeFamilies #-} import GHC.TypeLits data AB = A | B Double deriving (Show) class Foo x (a :: [Symbol]) where foo :: x -> AB instance (b ~ Double) => Foo b a where foo = B main = print $ foo 5 On Thu, Sep 22, 2016 at 6:39 PM, Tom Murphy wrote:

...

Bump -- is this feasible? It would be extremely helpful for an EDSL of mine.

Tom

On Tue, Aug 30, 2016 at 5:23 AM, Tom Murphy wrote:

...
I've got a case in a library I'm working on where having -XExtendedDefaultRules with MPTCs would be very, very helpful. Is it possible?

I.e., we can now write:

``` {-# LANGUAGE ExtendedDefaultRules #-}

data AB = A | B Double deriving (Show)

class Foo x where foo :: x -> AB

instance Foo Double where foo = B

main = print $ foo 5 ```

And -XExtendedDefaultRules makes sure we don't need to write "5 :: Double"

If, though, I want 'Foo' to take another parameter (here, a :: [Symbol]), it falls apart:

``` {-# LANGUAGE DataKinds #-} {-# LANGUAGE ExtendedDefaultRules #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE MultiParamTypeClasses #-}

import GHC.TypeLits

data AB = A | B Double deriving (Show)

class Foo x (a :: [Symbol]) where foo :: x -> AB

instance Foo Double a where foo = B

main = print $ foo 5 ```

Is there a reason MPTCs can't support ExtendedDefaultRules?

Thanks! Tom

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- Chris Allen Currently working on http://haskellbook.com

Tom Murphy

8:15 p.m.

With GHC 8.0.1 I get a type error: error: • Could not deduce (Foo x a0) from the context: Foo x a bound by the type signature for: foo :: Foo x a => x -> AB On Fri, Sep 23, 2016 at 8:56 AM, Christopher Allen wrote:

...

{-# LANGUAGE DataKinds #-} {-# LANGUAGE ExtendedDefaultRules #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TypeFamilies #-}

import GHC.TypeLits

data AB = A | B Double deriving (Show)

class Foo x (a :: [Symbol]) where foo :: x -> AB

instance (b ~ Double) => Foo b a where foo = B

main = print $ foo 5

On Thu, Sep 22, 2016 at 6:39 PM, Tom Murphy wrote:

...
Bump -- is this feasible? It would be extremely helpful for an EDSL of mine.

Tom

On Tue, Aug 30, 2016 at 5:23 AM, Tom Murphy wrote:

...
I've got a case in a library I'm working on where having -XExtendedDefaultRules with MPTCs would be very, very helpful. Is it possible?

I.e., we can now write:

``` {-# LANGUAGE ExtendedDefaultRules #-}

data AB = A | B Double deriving (Show)

class Foo x where foo :: x -> AB

instance Foo Double where foo = B

main = print $ foo 5 ```

And -XExtendedDefaultRules makes sure we don't need to write "5 ::

Double"

...
If, though, I want 'Foo' to take another parameter (here, a ::

[Symbol]),

...
it falls apart:

``` {-# LANGUAGE DataKinds #-} {-# LANGUAGE ExtendedDefaultRules #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE MultiParamTypeClasses #-}

import GHC.TypeLits

data AB = A | B Double deriving (Show)

class Foo x (a :: [Symbol]) where foo :: x -> AB

instance Foo Double a where foo = B

main = print $ foo 5 ```

Is there a reason MPTCs can't support ExtendedDefaultRules?

Thanks! Tom

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- Chris Allen Currently working on http://haskellbook.com

Tom Murphy

8:17 p.m.

Ah, looks like that's unrelated to the original issue, though -- thanks! Tom On Fri, Sep 23, 2016 at 9:15 AM, Tom Murphy wrote:

...

With GHC 8.0.1 I get a type error:

error: • Could not deduce (Foo x a0) from the context: Foo x a bound by the type signature for: foo :: Foo x a => x -> AB

On Fri, Sep 23, 2016 at 8:56 AM, Christopher Allen wrote:

...
{-# LANGUAGE DataKinds #-} {-# LANGUAGE ExtendedDefaultRules #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TypeFamilies #-}

import GHC.TypeLits

data AB = A | B Double deriving (Show)

class Foo x (a :: [Symbol]) where foo :: x -> AB

instance (b ~ Double) => Foo b a where foo = B

main = print $ foo 5

On Thu, Sep 22, 2016 at 6:39 PM, Tom Murphy wrote:

...
Bump -- is this feasible? It would be extremely helpful for an EDSL of mine.

Tom

On Tue, Aug 30, 2016 at 5:23 AM, Tom Murphy wrote:

...
I've got a case in a library I'm working on where having -XExtendedDefaultRules with MPTCs would be very, very helpful. Is it possible?

I.e., we can now write:

``` {-# LANGUAGE ExtendedDefaultRules #-}

data AB = A | B Double deriving (Show)

class Foo x where foo :: x -> AB

instance Foo Double where foo = B

main = print $ foo 5 ```

And -XExtendedDefaultRules makes sure we don't need to write "5 ::

Double"

...
If, though, I want 'Foo' to take another parameter (here, a ::

[Symbol]),

...
it falls apart:

``` {-# LANGUAGE DataKinds #-} {-# LANGUAGE ExtendedDefaultRules #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE MultiParamTypeClasses #-}

import GHC.TypeLits

data AB = A | B Double deriving (Show)

class Foo x (a :: [Symbol]) where foo :: x -> AB

instance Foo Double a where foo = B

main = print $ foo 5 ```

Is there a reason MPTCs can't support ExtendedDefaultRules?

Thanks! Tom

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- Chris Allen Currently working on http://haskellbook.com

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