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T3169.hs:13:22: error:
• Couldn't match type ‘elt’ with ‘Map b elt’
‘elt’ is a rigid type variable bound by
the type signature for:
lookup :: forall elt. (a, b) -> Map (a, b) elt -> Maybe elt
at T3169.hs:12:3-8
Expected type: Map a (Map b elt)
Actual type: Map (a, b) elt
• In the second argument of ‘lookup’, namely ‘m’
In the expression: lookup a m :: Maybe (Map b elt)
In the expression:
case lookup a m :: Maybe (Map b elt) of {
Just (m2 :: Map b elt) -> lookup b m2 :: Maybe elt }
• Relevant bindings include
m :: Map (a, b) elt (bound at T3169.hs:12:17)
b :: b (bound at T3169.hs:12:13)
lookup :: (a, b) -> Map (a, b) elt -> Maybe elt
(bound at T3169.hs:12:3)
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