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Type deduction panic

    • Closed Issue created by moomin @trac-moomin 
    > {-# LANGUAGE DeriveFunctor #-}

> module Bug where 

> import qualified Data.Set as S 

> import Data.Foldable (fold, foldMap, toList)
> import Data.List.Split (splitOn)
> import Data.Monoid -- ((<>), Sum, mempty, mappend)
> import Data.Functor.Foldable

> newtype Component = Component (Int,Int) deriving (Eq, Ord, Show)

> data Day24State v a = Day24State {
>   valueToMatch :: Int,
>   componentsToUse :: S.Set Component,
>   componentMetric :: v,
>   nextStates :: [a]     
> } deriving (Functor, Show)

> type Day24StateFix v a = Fix (Day24State v)

    Now in GHCI type

    :t Data.Functor.Foldable.ana f (undefined :: Day24StateFix Component)

    Produces

    ghc.EXE: panic! (the 'impossible' happened)

    (GHC version 8.0.2 for x86_64-unknown-mingw32):

    initTc: unsolved constraints WC {wc_insol = [W] f_akbJ :: t_akbI[tau:3] (CHoleCan: f)}

    Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug

    Trac metadata
    Trac field Value
    Version 8.2.2
    Type Bug
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
    Component Compiler
    Test case
    Differential revisions
    BlockedBy
    Related
    Blocking
    CC
    Operating system
    Architecture
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