[Haskell-cafe] Expressing seq

Bertram Felgenhauer bertram.felgenhauer at googlemail.com
Wed Sep 27 18:10:21 EDT 2006

Chad Scherrer wrote:
> Prelude> let sq x y = if x == x then y else y
> Prelude> 1 `sq` 2
> 2
> Prelude> (length [1..]) `sq` 2
> Interrupted.

> There must be a subtlety I'm missing, right?

Two, at least:

First, your sq has a different type, as it requires an Eq instance:

Prelude> :t sq
sq :: (Eq a) => a -> t -> t
Prelude> :t seq
seq :: a -> b -> b

Secondly, your sq is more akin to a deepSeq in that it forces all of
its value instead of just evaluating to weak head normal form.

Prelude> [undefined] `seq` 1
Prelude> [undefined] `sq` 1
*** Exception: Prelude.undefined

You could implement seq explicitely for many types, for example,

  seqList []    x = x
  seqList (_:_) x = x

but not for function types.



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