With a closed family, you could always do something like this:
type family Error (msg :: Symbol) -- no instances!
type family Sub a b where
-- real work
Sub a b = Error "Sub is undefined here"
Would that help? It still doesn't immediately error (there's no way to do
On Jun 19, 2015 2:29 PM, "Richard Eisenberg" wrote:
that currently), but it's perhaps more telling to have an error string.
Ghc does error out right away if you give it an infinite loop:
type family Fail (x::k) (a :: *) where
Fail x a = Fail x (Fail "don't raise the context stack please" a)
-- defining f leads to compile failure, and the message includes the "msg"
f :: Fail "msg" ()
f = undefined
But it would be nice to have a less hacky solution, especially as this one
gives error messages that suggest giving the type checker more resources.
Regards,
Adam
