[Haskell-cafe] Existentials and type var escaping

[David House]

On 31/05/07, Isaac Dupree <isaacdupree at charter.net> wrote:
> foo undefined = undefined

That's not true. When you evaluate foo undefined, it matches the first
(irrefutable) pattern immediately, without any deconstruction of the
undefined argument (which is the entire point of it being a lazy
pattern). So it becomes the right hand side, C <thunk>. Only when you
force that <thunk> would you have to force the undefined argument, so
foo undefined = C undefined:

*Main> foo undefined
C *** Exception: Prelude.undefined

-- 
-David House, dmhouse at gmail.com