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I just noticed while trying to learn to read GHC Core, that the automatically derived Eq instance for enum-style data types such as data EType = ETypeA | ETypeB | ETypeC | ETypeD | ETypeE | ETypeF | ETypeG | ETypeH deriving (Eq) seems to be transformed into a O(N) -like lookup when looking at GHC's core representation: $fEqEType_$c== = \ (a_ahZ :: EType) (b_ai0 :: EType) -> case a_ahZ of _ { ETypeA -> case b_ai0 of _ { ETypeA -> True; ETypeB -> False; ETypeC -> False; ETypeD -> False; ETypeE -> False; ETypeF -> False; ETypeG -> False; ETypeH -> False }; ETypeB -> case b_ai0 of _ {__DEFAULT -> False; ETypeB -> True}; ETypeC -> case b_ai0 of _ {__DEFAULT -> False; ETypeC -> True}; ETypeD -> case b_ai0 of _ {__DEFAULT -> False; ETypeD -> True}; ETypeE -> case b_ai0 of _ {__DEFAULT -> False; ETypeE -> True}; ETypeF -> case b_ai0 of _ {__DEFAULT -> False; ETypeF -> True}; ETypeG -> case b_ai0 of _ {__DEFAULT -> False; ETypeG -> True}; ETypeH -> case b_ai0 of _ {__DEFAULT -> False; ETypeH -> True} } Am I misinterpreting the GHC core output? Shouldn't algebraic data types provide an integer id for each constructor, which could then be compared directly in O(1) ? Also, why does the first case clause for ETypeA not make use of __DEFAULT as the other clauses do? update: As per suggestion by Simon Marlow, I addad a 9th constructor ETypeI , and then GHC switched to using dataToOtag# : $fEqEType_$c/= = \ (a_ahS :: EType) (b_ahT :: EType) -> case dataToTag# @ EType a_ahS of a#_ahQ { __DEFAULT ->

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