[Haskell-cafe] type level strings?

[Evan Laforge]

On Wed, Nov 23, 2011 at 4:03 PM, Holger Reinhardt <hreinhardt at gmail.com> 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...