Re: [Haskell-cafe] type level strings?

On Wed, Nov 23, 2011 at 4:03 PM, Holger Reinhardt wrote:

You can do this with phantom types, i.e.:

...

data Thing a = Thing Stuff

instance Monoid (Thing a) where   mappend (Thing stuff1) (Thing stuff2) = Thing (stuff1 `mappend` stuff2)   mempty = Thing mempty

data ID1 data ID2

thing1 :: Thing ID1 thing1 = Thing Stuff

thing2 :: Thing ID2 thing2 = Thing Stuff

-- will not typecheck: f = thing1 `mappend` thing2

Well yes, but the key feature is that the IDs are arbitrary strings. And they're not knowable at compile time, since they are read from user input...