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