[Haskell-cafe] seq and lambda with patterns
tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk
Wed Sep 28 12:28:18 UTC 2016
On Wed, Sep 28, 2016 at 03:00:57PM +0300, Denis Moskvin wrote:
> There is some subtle, but reasonable difference between \pat1 pat2 -> expr
> and \pat1 -> \pat2 -> expr:
> > seq ((\True y -> "DEFINED") undefined) 42
> > seq ((\True -> \y -> "DEFINED") undefined) 42
> *** Exception: Prelude.undefined
> The reason is the translation from Haskell Report 2010 (3.3)
> \ p1 . . . pn -> e = \ x1 . . . xn -> case (x1 , . . . , xn ) of (p1 , . .
> . , pn ) -> e
> Today I found (GHC 8.0.1) that
> > seq ((\True -> \y -> undefined) undefined) 42
> Is the last result correct?
According to the Haskell report this is
seq ((\x -> case x of True -> (\y -> undefined)) undefined) 42
and evaluation proceeds to
case undefined of True -> (\y -> undefined))
and thus to
So I agree with you that there's something fishy here.
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