Re: default instance for IsString

The defaulting is very good for most use cases, however I am discovering it won't default when I try to build up a list or tuple. This does not work: {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE ExtendedDefaultRules #-} {-# LANGUAGE FlexibleInstances #-} module Default (noDefault) where import Data.Text as T default (T.Text) class NoDefault a where noDefault :: a -> [Text] instance NoDefault [T.Text] where noDefault = id main = print (noDefault ["Hello!"]) On Sun, Apr 22, 2012 at 8:31 PM, Greg Weber wrote:

Sorry, someone responded on haskell-cafe and the message didn't get sent here. You can default a String. So this compiles just fine:

{-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE ExtendedDefaultRules #-} import Data.Text as T default (T.Text)

class    NoDefault a      where noDefault :: a -> Text instance NoDefault T.Text where noDefault = id

main = print (noDefault "Hello!")

On Sun, Apr 22, 2012 at 1:57 PM, Jeremy Shaw wrote:

...

I have often wished for something like:

{-# LANGUAGE StringLiteralsAs Text #-}

where all string literals like:

...

f = "foo"

would be translated to:

...

f = (fromString "foo" :: Text)

I find that OverloadedStrings is too general and causes ambiguous type errors. Additionally, I seldom find that I have more than one type of string literal per file. Things tend to be all String, all Text, etc. So, if I could just pick a concrete type for all the string literals in my file, I would be happy.

- jeremy

On Sat, Apr 21, 2012 at 7:20 PM, Greg Weber wrote:

...

I would like to default IsString to use the Text instance to avoid ambiguous type errors. I see defaulting capability is available for Num. Is there any way to do this for IsString?

Thanks, Greg Weber

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