is it possible to implement Functor for ByteString and Text

silvio

1 Mar 2015 1 Mar '15

3:11 a.m.

I have recently heard that some people want to burn bridges (introducing Foldable and Traversable to Prelude) and I've been wondering if it was possible somehow allow Text and Bytestring like containers to make use of those functions. Something along the lines of import qualified Data.ByteString as BS newtype ByteString' a = ByteString' BS.ByteString type ByteString = ByteString' Word8 instance (ByteString' a ~ ByteString' Word8) => Functor (ByteString') where fmap f (ByteString' bs) = ByteString' $ BS.map f bs Or if DataContexts worked as you would expect. newtype (Word8 ~ a) => ByteString' a = ByteString' BS.ByteString However I couldn't find a solution and I was just wondering if it is possible. P.S. Using GADTS it does actually work for Foldable, but only because it doesn't have to output any ByteStrings. It doesn't work for Functor for instance. data ByteString' a where ByteString' :: BS.ByteString -> ByteString' Word8 type ByteString = ByteString' Word8 instance Foldable ByteString' where foldr f ini (ByteString' bs) = BS.foldr f ini bs Silvio

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David Feuer

1 Mar 1 Mar

3:41 a.m.

New subject: is it possible to implement Functor for ByteString and Text

This is impossible. The type of fmap is fmap :: Functor f => (a->b) -> f a -> f b You can use a GADT to effectively restrict what a is, because the caller won't be able to provide a non-bottom f a if a has the wrong type. But the caller can choose absolutely any type for b, and there's nothing you can do about that. David On Sat, Feb 28, 2015 at 2:11 PM, silvio wrote:

...

I have recently heard that some people want to burn bridges (introducing Foldable and Traversable to Prelude) and I've been wondering if it was possible somehow allow Text and Bytestring like containers to make use of those functions. Something along the lines of

import qualified Data.ByteString as BS

newtype ByteString' a = ByteString' BS.ByteString

type ByteString = ByteString' Word8

instance (ByteString' a ~ ByteString' Word8) => Functor (ByteString') where fmap f (ByteString' bs) = ByteString' $ BS.map f bs

Or if DataContexts worked as you would expect.

newtype (Word8 ~ a) => ByteString' a = ByteString' BS.ByteString

However I couldn't find a solution and I was just wondering if it is possible.

P.S. Using GADTS it does actually work for Foldable, but only because it doesn't have to output any ByteStrings. It doesn't work for Functor for instance.

data ByteString' a where ByteString' :: BS.ByteString -> ByteString' Word8

type ByteString = ByteString' Word8

instance Foldable ByteString' where foldr f ini (ByteString' bs) = BS.foldr f ini bs

Silvio _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

Christopher Done

3:56 a.m.

New subject: is it possible to implement Functor for ByteString and Text

You could have class IsWord8 a instance IsWord8 Word8 instance (IsWord8 a) => Functor (ByteString a) where ... It would be a legitimate instance. On 28 February 2015 at 20:41, David Feuer wrote:

...

This is impossible. The type of fmap is

fmap :: Functor f => (a->b) -> f a -> f b

You can use a GADT to effectively restrict what a is, because the caller won't be able to provide a non-bottom f a if a has the wrong type. But the caller can choose absolutely any type for b, and there's nothing you can do about that.

David

On Sat, Feb 28, 2015 at 2:11 PM, silvio wrote:

...
I have recently heard that some people want to burn bridges (introducing Foldable and Traversable to Prelude) and I've been wondering if it was possible somehow allow Text and Bytestring like containers to make use of those functions. Something along the lines of

import qualified Data.ByteString as BS

newtype ByteString' a = ByteString' BS.ByteString

type ByteString = ByteString' Word8

instance (ByteString' a ~ ByteString' Word8) => Functor (ByteString') where fmap f (ByteString' bs) = ByteString' $ BS.map f bs

Or if DataContexts worked as you would expect.

newtype (Word8 ~ a) => ByteString' a = ByteString' BS.ByteString

However I couldn't find a solution and I was just wondering if it is possible.

P.S. Using GADTS it does actually work for Foldable, but only because it doesn't have to output any ByteStrings. It doesn't work for Functor for instance.

data ByteString' a where ByteString' :: BS.ByteString -> ByteString' Word8

type ByteString = ByteString' Word8

instance Foldable ByteString' where foldr f ini (ByteString' bs) = BS.foldr f ini bs

Silvio _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

Christopher Done

3:58 a.m.

New subject: is it possible to implement Functor for ByteString and Text

Ah, I'm talking nonsense. Ignore me. On 28 February 2015 at 20:56, Christopher Done wrote:

...

You could have class IsWord8 a instance IsWord8 Word8 instance (IsWord8 a) => Functor (ByteString a) where ...

It would be a legitimate instance.

On 28 February 2015 at 20:41, David Feuer wrote:

...
This is impossible. The type of fmap is

fmap :: Functor f => (a->b) -> f a -> f b

You can use a GADT to effectively restrict what a is, because the caller won't be able to provide a non-bottom f a if a has the wrong type. But the caller can choose absolutely any type for b, and there's nothing you can do about that.

David

On Sat, Feb 28, 2015 at 2:11 PM, silvio wrote:

...
I have recently heard that some people want to burn bridges (introducing Foldable and Traversable to Prelude) and I've been wondering if it was possible somehow allow Text and Bytestring like containers to make use of those functions. Something along the lines of

import qualified Data.ByteString as BS

newtype ByteString' a = ByteString' BS.ByteString

type ByteString = ByteString' Word8

instance (ByteString' a ~ ByteString' Word8) => Functor (ByteString') where fmap f (ByteString' bs) = ByteString' $ BS.map f bs

Or if DataContexts worked as you would expect.

newtype (Word8 ~ a) => ByteString' a = ByteString' BS.ByteString

However I couldn't find a solution and I was just wondering if it is possible.

P.S. Using GADTS it does actually work for Foldable, but only because it doesn't have to output any ByteStrings. It doesn't work for Functor for instance.

data ByteString' a where ByteString' :: BS.ByteString -> ByteString' Word8

type ByteString = ByteString' Word8

instance Foldable ByteString' where foldr f ini (ByteString' bs) = BS.foldr f ini bs

Silvio _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

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Chris Wong

8:01 a.m.

New subject: is it possible to implement Functor for ByteString and Text

On Sun, Mar 1, 2015 at 8:11 AM, silvio wrote:

...

I have recently heard that some people want to burn bridges (introducing Foldable and Traversable to Prelude) and I've been wondering if it was possible somehow allow Text and Bytestring like containers to make use of those functions. Something along the lines of

import qualified Data.ByteString as BS

newtype ByteString' a = ByteString' BS.ByteString

type ByteString = ByteString' Word8

instance (ByteString' a ~ ByteString' Word8) => Functor (ByteString') where fmap f (ByteString' bs) = ByteString' $ BS.map f bs

If tweak the definition of Functor a bit, we can get that to work: {-# LANGUAGE ConstraintKinds, TypeFamilies #-} import qualified Data.ByteString as B import Data.Word (Word8) import GHC.Prim (Constraint) newtype ByteString' a = ByteString' B.ByteString deriving (Eq, Ord, Show) class Functor' f where type FunctorConstraint f a :: Constraint fmap' :: (FunctorConstraint f a, FunctorConstraint f b) => (a -> b) -> f a -> f b instance Functor' ByteString' where type FunctorConstraint ByteString' a = a ~ Word8 fmap' f (ByteString' x) = ByteString' $ B.map f x But I don't think it's possible with the original type class. -- https://lambda.xyz

silvio

9:08 a.m.

New subject: is it possible to implement Functor for ByteString and Text

Wow ConstraintKinds. There's always a new extension to be learned :) Anyway, if changing the Functor declaration were allowed, it would probably make more sense to use something like MonoFunctor. Unfortunately, MPTC or type family stuff is never going to make it into Prelude. Silvio

Niklas Haas

10:58 a.m.

New subject: is it possible to implement Functor for ByteString and Text

I think it's more realistic to use lens style Setters where possible. Essentially: type Setter s t a b = (a -> b) -> s -> t type Setter' s a = Setter s s a a bytes :: Setter ByteString Word8 bytes = BS.map fmapped :: Functor f => Setter (f a) (f b) a b fmapped = fmap In this framework, you could write a function that can abstract over any setter, eg. changeSomething :: Setter s t Foo Bar -> s -> t changeSomething s = s fooBar where fooBar :: Foo -> Bar It's not quite the same thing as making ByteString or Text an instance of Functor, but for some tasks, it can be a good replacement. On Sun, 01 Mar 2015 02:08:40 +0100, silvio wrote:

...

Wow ConstraintKinds. There's always a new extension to be learned :) Anyway, if changing the Functor declaration were allowed, it would probably make more sense to use something like MonoFunctor. Unfortunately, MPTC or type family stuff is never going to make it into Prelude.

Silvio _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

Zemyla

2 Mar 2 Mar

2:06 p.m.

New subject: is it possible to implement Functor for ByteString and Text

What I would do is hold the function to apply in the wrapper type. import qualified Data.ByteString as BS data ByteString' a = ByteString' (Word8 -> a) BS.ByteString wrap :: BS.ByteString -> ByteString' Word8 wrap bs = ByteString' id bs -- The type ensures you can only unwrap with a function Word8 -> Word8. unwrap :: ByteString' Word8 -> ByteString unwrap (ByteString' f bs) = BS.map f bs -- Functor instance just concatenates the fmapped function. instance Functor ByteString' where fmap f (ByteString' g bs) = ByteString' (f . g) bs -- Foldable instance just uses the fmapped function. instance Foldable ByteString' where foldr f z (ByteString' g bs) = BS.foldr (f . g) z bs -- You could define foldr', foldl, etc. based on the ones in Data.ByteString. -- Not strictly necessary, but nice to have. As an added benefit, this doesn't require GADTs. It probably would if you wanted to implement Monad as well, though. On Feb 28, 2015 1:11 PM, "silvio" wrote:

...

I have recently heard that some people want to burn bridges (introducing Foldable and Traversable to Prelude) and I've been wondering if it was possible somehow allow Text and Bytestring like containers to make use of those functions. Something along the lines of

import qualified Data.ByteString as BS

newtype ByteString' a = ByteString' BS.ByteString

type ByteString = ByteString' Word8

instance (ByteString' a ~ ByteString' Word8) => Functor (ByteString') where fmap f (ByteString' bs) = ByteString' $ BS.map f bs

Or if DataContexts worked as you would expect.

newtype (Word8 ~ a) => ByteString' a = ByteString' BS.ByteString

However I couldn't find a solution and I was just wondering if it is possible.

P.S. Using GADTS it does actually work for Foldable, but only because it doesn't have to output any ByteStrings. It doesn't work for Functor for instance.

data ByteString' a where ByteString' :: BS.ByteString -> ByteString' Word8

type ByteString = ByteString' Word8

instance Foldable ByteString' where foldr f ini (ByteString' bs) = BS.foldr f ini bs

Silvio _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

silvio

4 Mar 4 Mar

12:46 a.m.

New subject: is it possible to implement Functor for ByteString and Text

cool trick. This is by far the best solution yet. Of course it's a bit deceptive in what you are working with. E.g. bs1 <- pack [1..10] print bs1 let bs2 = map (+1) bs1 print bs2 let bs3 = map (+1) bs2 print bs3 ... let bsn = map (+1) bsn_1 print bsn will have quadratic complexity. On the other, hand you will get fusion for free. silvio

Ben Franksen

6 Mar 6 Mar

12:47 p.m.

New subject: is it possible to implement Functor for ByteString and Text

silvio wrote:

...

cool trick. This is by far the best solution yet. Of course it's a bit deceptive in what you are working with. E.g.

bs1 <- pack [1..10] print bs1 let bs2 = map (+1) bs1 print bs2 let bs3 = map (+1) bs2 print bs3 ... let bsn = map (+1) bsn_1 print bsn

will have quadratic complexity.

One could perhaps replace id with unsafeCoerce in wrap? Cheers Ben

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participants (7)

Ben Franksen
Chris Wong
Christopher Done
David Feuer
Niklas Haas
silvio
Zemyla