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Type classes for converting to Text and String

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Simon Hengel

8 Mar 2012 8 Mar '12

4:53 a.m.

Hi! When writing library code that should work with both String and Text I find my self repeatedly introducing classes like: class ToString a where toString :: a -> String class ToText a where toText :: a -> Text (I use this with newtype wrapped value types backed by Text or ByteString.) So I wonder whether it would be a good idea to have a package that provides those classes. Or maybe just ToText, and provide default implementations of toString and toText, like: class ToText a where toText :: a -> Text toText = Text.pack . toString toString :: a -> String toString = Text.unpack . toText How do you guys deal with that? Any thoughts? Cheers, Simon

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Christopher Done

8 Mar 8 Mar

5 a.m.

On 8 March 2012 10:53, Simon Hengel wrote:

When writing library code that should work with both String and Text I find my self repeatedly introducing classes like:

class ToString a where toString :: a -> String

class ToText a where toText :: a -> Text

Text is already an instance of IsString which provides IsString. I've defined ToString in my own projects though, it would be nice for it to be defined somewhere (Data.String maybe?).

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Simon Hengel

5:42 a.m.

On Thu, Mar 08, 2012 at 11:00:34AM +0100, Christopher Done wrote:

On 8 March 2012 10:53, Simon Hengel wrote:

...
When writing library code that should work with both String and Text I find my self repeatedly introducing classes like:

class ToString a where toString :: a -> String

class ToText a where toText :: a -> Text

Text is already an instance of IsString which provides IsString.

What exactly do you mean?

I've defined ToString in my own projects though, it would be nice for it to be defined somewhere (Data.String maybe?).

We could write a proposal to add ToString to base (maybe a good idea, not sure). ToString has a striking similarity with Show, but it's still different: * toString converts some a to a String * show gives a string _representation_ of some a (e.g. converting a String to a String is just id and hence different from show; this is akin to Python's __str__/__repr__) But this does still not help with toText. Cheers, Simon

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Roman Cheplyaka

5:18 a.m.

* Simon Hengel [2012-03-08 10:53:15+0100]

When writing library code that should work with both String and Text I find my self repeatedly introducing classes like: [...] How do you guys deal with that? Any thoughts?

If it's fine to depend on FunDeps, you can use ListLike. http://hackage.haskell.org/package/ListLike -- Roman I. Cheplyaka :: http://ro-che.info/

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Simon Hengel

5:48 a.m.

On Thu, Mar 08, 2012 at 12:18:56PM +0200, Roman Cheplyaka wrote:

If it's fine to depend on FunDeps, you can use ListLike. http://hackage.haskell.org/package/ListLike

How would that help with toText? Cheers, Simon

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Roman Cheplyaka

5:54 a.m.

* Simon Hengel [2012-03-08 11:48:41+0100]

On Thu, Mar 08, 2012 at 12:18:56PM +0200, Roman Cheplyaka wrote:

...
If it's fine to depend on FunDeps, you can use ListLike. http://hackage.haskell.org/package/ListLike

How would that help with toText?

toText = fromListLike (ListLike instance for Text is provided by the listlike-instances package.) -- Roman I. Cheplyaka :: http://ro-che.info/

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Simon Hengel

7:20 a.m.

On Thu, Mar 08, 2012 at 12:54:13PM +0200, Roman Cheplyaka wrote:

* Simon Hengel [2012-03-08 11:48:41+0100]

...
On Thu, Mar 08, 2012 at 12:18:56PM +0200, Roman Cheplyaka wrote:

...
If it's fine to depend on FunDeps, you can use ListLike. http://hackage.haskell.org/package/ListLike

How would that help with toText?

toText = fromListLike

(ListLike instance for Text is provided by the listlike-instances package.)

Ah, the listlike-instances package is the missing piece. Not sure if this is going somewhere. But I'm still trying to get a clear picture of the performance implications. Say I have a newtype-wrapped ByteString that I would decode to String/Text using UTF-8: newtype Value = Value ByteString Would it be possible to go from Value to Text by essentially ending up with Data.Text.Encoding.decodeUtf8 at runtime (e.g. by using rewrite rules)? Cheers, Simon

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Roman Cheplyaka

7:40 a.m.

* Simon Hengel [2012-03-08 13:20:22+0100]

On Thu, Mar 08, 2012 at 12:54:13PM +0200, Roman Cheplyaka wrote:

...
* Simon Hengel [2012-03-08 11:48:41+0100]

...
On Thu, Mar 08, 2012 at 12:18:56PM +0200, Roman Cheplyaka wrote:

...
If it's fine to depend on FunDeps, you can use ListLike. http://hackage.haskell.org/package/ListLike

How would that help with toText?

toText = fromListLike

(ListLike instance for Text is provided by the listlike-instances package.)

Ah, the listlike-instances package is the missing piece.

Not sure if this is going somewhere. But I'm still trying to get a clear picture of the performance implications.

Say I have a newtype-wrapped ByteString that I would decode to String/Text using UTF-8:

newtype Value = Value ByteString

Would it be possible to go from Value to Text by essentially ending up with Data.Text.Encoding.decodeUtf8 at runtime (e.g. by using rewrite rules)?

You can do that, but it will work only if your functions are specialized enough at compile time. -- Roman I. Cheplyaka :: http://ro-che.info/

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Yves Parès

6:37 a.m.

If you just need to go back and forth from String to Text, why do you need to be generic? pack and unpack from Data.Text do the job. Plus, in the way of what Christopher said, you can use the OverloadedStrings extension. You can then use the string syntax at a place that expects a text: {-# LANGUAGE OverloadedStrings #-} import Data.Text t :: Text t = "Hello" Any instance of the IsString class can be used in this way, not only Text. 2012/3/8 Simon Hengel

Hi!

When writing library code that should work with both String and Text I find my self repeatedly introducing classes like:

class ToString a where toString :: a -> String

class ToText a where toText :: a -> Text

(I use this with newtype wrapped value types backed by Text or ByteString.)

So I wonder whether it would be a good idea to have a package that provides those classes.

Or maybe just ToText, and provide default implementations of toString and toText, like:

class ToText a where

toText :: a -> Text toText = Text.pack . toString

toString :: a -> String toString = Text.unpack . toText

How do you guys deal with that? Any thoughts?

Cheers, Simon

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Simon Hengel

6:59 a.m.

On Thu, Mar 08, 2012 at 12:37:31PM +0100, Yves Parès wrote:

If you just need to go back and forth from String to Text, why do you need to be generic? pack and unpack from Data.Text do the job.

Always going through String or Text may (depending on what your underlying representation is) be less efficient than converting directly to String/Text. Cheers, Simon

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participants (4)

Christopher Done
Roman Cheplyaka
Simon Hengel
Yves Parès