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T2627b.hs:20:24: error: [GHC-27958]
• Could not deduce ‘Dual (Dual a0) ~ a0’
arising from a use of ‘conn’
from the context: (Dual a ~ b, Dual b ~ a)
bound by the type signature for:
conn :: forall a b.
(Dual a ~ b, Dual b ~ a) =>
Comm a -> Comm b -> (Int, Int)
at T2627b.hs:19:1-66
or from: a ~ R c d
bound by a pattern with constructor:
Rd :: forall c d. (c -> Comm d) -> Comm (R c d),
in an equation for ‘conn’
at T2627b.hs:20:7-10
or from: b ~ W e f
bound by a pattern with constructor:
Wr :: forall e f. e -> Comm f -> Comm (W e f),
in an equation for ‘conn’
at T2627b.hs:20:14-19
The type variable ‘a0’ is ambiguous
• In the expression: conn undefined undefined
In an equation for ‘conn’:
conn (Rd k) (Wr a r) = conn undefined undefined
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