> Wait, a pattern match can have side effects? Can you provide some
> example code? (do you mean, pattern match failure / exceptions / run
> time errors?)
>
> I'm new to Haskell, but in my understanding, it is said that Haskell
> is pure because the whole Haskell code is just specification of
> (types, .. and) values, and this specification is enjoy referential
> transparency.
There are two definition of "pure".

1°) a purely functional language without any side effect (no exception
or error)
Haskell do have partial match allowed, producing a "bottom" value and it
also have non terminating program. For this definition, I think there
are no pure language except may be languages used in theorem prover
where totality is a requirement.

2°) enjoying referential transparency but alowwing side effects in a
controlled way : you can list all possible side effects and have good
way to predict them (the compiler warns you that side effect are possible).

Actually very few language implement a termination checker to warn you
about possible non terminating recursive definition in your code ...
This means that currently non termination is not really considered as a
side effect which allows to say that haskell is "pure" for the second
definition.

Cheers,
Christophe

-- 
Christophe Raffalli
Universite de Savoie
Batiment Le Chablais, bureau 21
73376 Le Bourget-du-Lac Cedex

tel: (33) 4 79 75 81 03
fax: (33) 4 79 75 87 42
mail: Christophe.Raffalli@univ-savoie.fr
www: http://www.lama.univ-savoie.fr/~RAFFALLI
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