Re: patch applied (packages/base): IsString is GHC-only (so why is it in the Prelude?)

I'm sorry for putting IsString into Base like that. Like Ian I don't know where else it should go, and since the module is named GHC.Base the name seems to indicate that what is in it is for GHC, so I didn't think it needed to have any #ifdef around it. Would some #ifdef somewhere be appropriate? -- Lennart On Jan 24, 2007, at 08:15 , Ian Lynagh wrote:

On Tue, Jan 23, 2007 at 06:43:22PM +0000, Duncan Coutts wrote:

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On Tue, 2007-01-23 at 10:38 -0800, Ross Paterson wrote:

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Tue Jan 23 10:30:07 PST 2007 Ross Paterson * IsString is GHC-only (so why is it in the Prelude?)

Yes, can we please have a discussion about what's going on here. Changes to the base library are supposed to go via the libraries review process. I'm not complaining that we slipped up, I just want to make sure people can have a chance to comment. I'm cc'ing the libraries list.

For those who haven't been following, this comes from a new GHC extension that means string literals like "foo" have type (IsString a => a) rather than String. This is analogous to (5 :: Num a => a) in Haskell 98.

This uses a new class

class IsString a where fromString :: String -> a

which is defined in GHC.Base and re-exported by the Prelude.

Letting this Prelude change through was my fault - for some reason I didn't notice it, or it didn't register, or something.

That said, I'm not sure where else I'd put it. GHC.Exts is the only place I can think of that it vaguely belongs, but the class itself is entirely portable, so this doesn't feel right to me.

Also, in GHC(-fglasgow-exts) the class will appear in types with no other modules loaded. If the extension was in the language then I would certainly expect the class to be in the Prelude.

When I last asked Isaac about it, it hadn't beeen decided how much Haskell' will say about libraries. I can't remember if I've said this in public before, but my opinion is that it should say as little as possible; we have recently been able to make a number of improvements to the base libraries as the move to hierarchical libraries has meant that the Haskell 98 standard hasn't tied our hands, and I think it would be a mistake to tie them again with Haskell'. Thus I think the addition of such a class, even to the Prelude, should be OK for Haskell'. Of course, concensus may not agree with me, and it's also possible there will be releases of implementations using the HEAD branch of the base packages before Haskell' is done.

So to summarise, it was a mistake to let it in without discussion; sorry. But my opinion is that it should be there (for all implementations) anyway once we are making Haskell' implementations, and I don't know where to put it in the meantime.

Thanks Ian