Ben Gamari <b...@smart-cactus.org> writes: > Simon Peyton Jones <simo...@microsoft.com> writes: > >> Meanwhile to get you rolling, you can replace rewritableTyVars with >> tyCoVarsOfType and it'll all work, just a bit less efficiently. >> > Thanks Simon! Indeed that allows things to proceed and in the process, > confirms that there is a bug in my solver logic. > I should clarify: the bug is in fact not in the Typeable solver. The problem is that we have a type with some kind polymorphism. For instance,
data Proxy (a :: k) = Proxy
Note that in order to be `Typeable` Proxy needs to have its `k` kind
argument instantiated. Consequently this program is perfectly fine,
ty :: TypeRep (Proxy Int)
ty = typeRep
We can even decompose this into a type application,
case ty of
TRApp a b -> ...
where `a :: TypeRep Proxy` (with `k ~ Type`) and `b :: TypeRep Int`.
However, if we attempt to decompose `a` again (which is what the
Typeable1 testcase described in this thread tests), we run into trouble,
case a of
TRApp x y -> ...
After changing rewritableTyVars with tyCoVarsOfType, the testcase
Typeable1 fails with the following correct, albeit not terribly
readable, error message,
Typeable1.hs:22:5: error:
• Couldn't match kind ‘* -> (* -> *) -> (* -> *) -> * -> *’
with ‘forall k. (* -> *) -> (k -> *) -> k -> *’
Inaccessible code in
a pattern with pattern synonym:
TRApp :: forall k2 (t :: k2).
() =>
forall k1 (a :: k1 -> k2) (b :: k1).
t ~ a b =>
TypeRep (k1 -> k2) a -> TypeRep k1 b -> TypeRep k2 t,
in a pattern binding in
'do' block
• In the pattern: TRApp x y
In a stmt of a 'do' block: TRApp x y <- pure x
In the expression:
do let x :: ComposeK Maybe Maybe Int
x = undefined
TRApp x y <- pure $ typeOf x
print (x, y)
TRApp x y <- pure x
....
• Relevant bindings include
y :: TypeRep k3 b2 (bound at Typeable1.hs:19:13)
x :: TypeRep (k3 -> k2 -> k1 -> *) a2 (bound at Typeable1.hs:19:11)
This might be a place where GHC could hold the user's hand a bit more gently.
Anyways, hopefully this will be resolved with the fix to Simon's second
issue. Otherwise I just need to look at two performance regressions and
I think I'm done.
Cheers,
- Ben
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