consider
data m @ a => T m a = {.. }
and a top level definition
qqqq :: T m a -> ...
qqqq (T f) = ...
now we have the following equations:
T @ m ~ ()
T m @ a ~ m @ a
Use of T will give rise to a m @ a constraint and elaboration of T m a will give us T @ m, T m @ a
however GHC solver is not smart enough to figure out that m @ a can be solved using T m @ a
This means we need to give an explicit v @ a constraint in qqqq for it to typecheck
consider
and a top level definition
now we have the following equations:
Use of
Twill give rise to am @ aconstraint and elaboration ofT m awill give usT @ m, T m @ ahowever GHC solver is not smart enough to figure out that
m @ acan be solved usingT m @ aThis means we need to give an explicit
v @ aconstraint inqqqqfor it to typecheck