String != [Char]

ARJANEN Loïc Jean David arjanen.loic at
Wed Mar 21 18:46:14 CET 2012

Le 20/03/2012 16:29, Tillmann Rendel a écrit :
> Hi,
> Thomas Schilling wrote:
>> I agree that the language standard should not prescribe the
>> implementation of a Text datatype.  It should instead require an
>> abstract data type (which may just be a newtype wrapper for [Char] in
>> some implementations) and a (minimal) set of operations on it.
>> Regarding the type class for converting to and from that type, there
>> is a perhaps more complicated question: The current fromString method
>> uses String as the source type which causes unnecessary overhead.
> Is this still a problem if String would be replaced by an 
> implementation-dependend newtype? Presumably, GHC would use a more 
> efficient representation behind the newtype, so the following would be 
> efficient in practice (or not?)
>   newtype String
>     = ...
>   class IsString a where
>     fromString :: String -> a
> The standard could even prescribe that an instance for [Char] exists:
>   explode :: String -> [Char]
>   explode = ...
>   instance IsString [Char] where
>     fromString = explode
> Tillmann

A recent message on Haskell-café made me think that if the standard 
mandates that any instance exists, it should mandates that an instance 
exists for CString and CWString (C's strings and wide strings) or, more 
generally, that an instance exists for any foreign string type defined 
in the FFIs implemented. That is to say, if you implement a FFI for .Net 
and you expose .Net's string type, you should implement conversions 
between that string type and Haskell's one.


More information about the Haskell-prime mailing list