[Haskell-cafe] On the purity of Haskell

[Heinrich Apfelmus]

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