How does seq force functions?

Background This question arises from a challenge Brent Yorgey posed at OPLSS: write a function f :: (Int -> Int) -> Bool that distinguishes f undefined from f (\x -> undefined). All of our answers either used seq or something like bang patterns that desugar into seq. For example: f :: (Int -> Int) -> Bool f g = g `seq` True *Main> f undefined *** Exception: Prelude.undefined *Main> f (\x -> undefi

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"seq cannot be implemented in Haskell." by Don