Steve Horne wrote:
This is just my view on whether Haskell is pure, being offered up for
criticism. I haven't seen this view explicitly articulated anywhere
before, but it does seem to be implicit in a lot of explanations - in
particular the description of Monads in SBCs "Tackling the Awkward
Squad". I'm entirely focused on the IO monad here, but aware that it's
just one concrete case of an abstraction.
Warning - it may look like trolling at various points. Please keep going
to the end before making a judgement.
To make the context explicit, there are two apparently conflicting
viewpoints on Haskell...
1. The whole point of the IO monad is to support programming with
side-effecting actions - ie impurity.
2. The IO monad is just a monad - a generic type (IO actions), a couple
of operators (primarily return and bind) and some rules - within a
pure functional language. You can't create impurity by taking a
subset of a pure language.
My view is that both of these are correct, each from a particular point
of view. Furthermore, by essentially the same arguments, C is also both
an impure language and a pure one. [...]
Purity has nothing to do with the question of whether you can express IO
in Haskell or not.
The word "purity" refers to the fact that applying a value
foo :: Int -> Int
(a "function") to another value *always* evaluates to the same result.
This is true in Haskell and false in C.
The beauty of the IO monad is that it doesn't change anything about
purity. Applying the function
bar :: Int -> IO Int
to the value 2 will always give the same result:
bar 2 = bar (1+1) = bar (5-3)
Of course, the point is that this result is an *IO action* of type IO
Int , it's not the Int you would get "when executing this action".
Best regards,
Heinrich Apfelmus
--
http://apfelmus.nfshost.com
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