Chad Scherrer wrote:
I was reading on p. 29 of "A History of Haskell" (a great read, by the way) about the controversy of adding seq to the language. But other than for efficiency reasons, is there really any new primitive that needs to be added to support this?
As long as the compiler doesn't optimize it away, why not just do something like this (in ghci)?
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?
The (sq x) function depends on x being an instance of typeclass Eq. Imagine a new typeclass Seq that is auto-defined for all types: class Seq a where seq :: a -> (b -> b) data Foo x = Bar x | Baz | Foo y x The instances always use a "case" to force just enough evaluation to compute the constructor, then return id: instance Seq Foo where seq a = case a of Bar _ -> id Baz -> id Foo _ _ -> id foo1,foo2 :: Foo Int foo1 = undefined foo2 = Bar 1 Now "(seq foo1) b" will also be undefined which "(seq foo2) b" will be "id b" which is "b".