This seems unintended. Maybe others are wise to the contrary? Perhaps a patch should be made. Thanks On Mon, Apr 13, 2020, 1:57 PM Christiaan Baaij wrote:

Hello list,

Is the following behavior intended? or an oversight?

Prelude Control.DeepSeq> tail ((undefined :: ()) `seq` 1 : []) *** Exception: Prelude.undefined CallStack (from HasCallStack): error, called at libraries/base/GHC/Err.hs:80:14 in base:GHC.Err undefined, called at <interactive>:5:8 in interactive:Ghci2 Prelude Control.DeepSeq> tail ((undefined :: ()) `deepseq` 1 : []) []

If you're wondering what's happening, there is a (builtin) fixity declaration for 'seq': infixr `seq` 0

But there isn't one for `deepseq`.

So

undefined `seq` 1 : []

is parsed as

undefined `seq` (1 : [])

while

undefined `deepseq` 1 : []

is parsed as

(undefined `deepseq` 1) : []

In a real-life use-case, changing `seq` to `deepseq` actually resulted in a (very unanticipated) space-leak for me. So again, is the lack of fixity declaration for deepseq intentional? or an accidental omission?

Best regards,

Christiaan

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