haskell prelude : definition of seq

Question

In the definition of the haskell prelude we see that ... is reserved for expressions that cannot be implemented in Haskell. Now the IO monad for example can't be implemented in haskell.

What surprised me is that seq is defined as follows in the prelude

seq :: a -> b -> b
seq = ...       -- Primitive

Why not the following? what am i missing?

seq _ b = b

Answer 1

As you can see in the Haskell Wiki entry on seq , the seq function must fulfil the following two equations:

⊥ `seq` b = ⊥
a `seq` b = b

(where ⊥ is the undefined value, which is what non-terminating function applications or things like hd [] and undefined evaluate to logically)

Your definition clearly does not fulfil the first equation.

The typical use case for seq is to force evaluation of the first parameter (to weak head normal form) before evaluating the second one. (although, strictly speaking, seq does not guarantee that; see again the wiki article)

Such a function is, to my knowledge, not definable in pure Haskell without any compiler extensions like -XBangPatterns .