[Haskell-beginners] Re: homogeneous value list

Heinrich Apfelmus apfelmus at quantentunnel.de
Fri Mar 12 12:00:10 EST 2010

Thomas Davie wrote:
> Salil Wadnerkar wrote:
>> Ozgur Akgun wrote:
>>> Another boring variant from me then.
>>> isHomogeneous xs = all (first==) xs
>>>     where first = head xs
>> Shouldn't
>> head xs
>> give an exception on an empty list?
> An error, and only if it's evaluated.  Lazy evaluation means it's not evaluated here.
> Of course another non-strict algorithm *might* evaluate head xs here,
> so this version won't work in all possible Haskell implementations,
> only the current ones which use lazy evaluation.

Nope, this works for any non-strict implementation. It can be deduced
from the Haskell98 language standard that  all  has the property

    all ⊥ [] = True

(Non-strict semantics doesn't tell you anything about time and space
behavior, though; that's where you need the information that the
compiler is using lazy evaluation.)

Heinrich Apfelmus


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