Disclaimer: this use case for type-level string ops is still hypothetical, so these are predictions. Shooting for the moon, I foresee writing a type-level string similarity metric. In my experience, that would involve nested traversals, sliding of sequence windows, etc. In that case, I would very much like to write the similarity measure algorithm as if the labels were a sequence of characters. The kind in the following signature assumes promoted lists and — though I know it's still up in the air — some sort of promotion of characters as Char.
type LabelToLetters :: Label -> [Char]
Alternatively, the Char kind could simply be Label with an extralingual invariant that it has one "character" in it — though that doesn't smell like Haskell. The resulting kind of LabelToLetters in this case would be (Label -> [Label]). A class of "dumber" algorithms for "comparing" type-level promotions of constructor names might simply attach some prefix or suffix and then test for equality. In that case, I'd just need append.
type LabelAppend :: Label -> Label -> Label
I personally lean towards LabelToLetters, since I predict that any direct interface for labels-as-sequences is going to perpetually be a subset of whatever becomes established as the interface for type-level lists. The symmetric LettersToLabel would be nice for balance, but I don't foresee needing it in this use case, since comparing Labels is more so a consumer of Labels as opposed to a producer. Whatever the interface to Labels, I would need to be able to test type-level characters for equality. While built-in decidable type equality family that returns a type of kind Bool would be fantastic (or preferably GHC ticket 1894), I'm not anticipating that anytime soon. In order to continue using my own library for such a type-level type equality predicate, I need to be able to provide type family instances for the type-level characters.
I need type-level characters to be valid as indices in type family instances.
That's all I foresee. Thanks.
On Tue, Jan 3, 2012 at 3:32 AM, Simon Peyton-Jones
| In regard to Labels versus Atom, etc., in my use case of converting | between similar datatypes, it would be very reasonable to eventually | add/remove prefixes/suffixes from these type-level reifications of | constructor names. If type-level strings are not implemented as lists | of characters, I would still like access to a comparable API.
Can you be specific? What operations, exactly, do you want?
Simon