[Haskell-cafe] Re: Can we come out of a monad?

[Felipe Lessa]

On Tue, Aug 10, 2010 at 6:21 PM, Martijn van Steenbergen
<martijn at van.steenbergen.nl> wrote:
> On 8/2/10 7:09, Ertugrul Soeylemez wrote:
>>
>> Given the definition of a Haskell function, Haskell is a pure language.
>> The notion of a function in other languages is not:
>>
>>   int randomNumber();
>>
>> The result of this function is an integer.  You can't replace the
>> function call by its result without changing the meaning of the program.
>
> I'm not sure this is fair. It's perfectly okay to replace a call
> "randomNumber()" by that method's *body* (1), which is what you argue is
> okay in Haskell.

Nope.  For example, suppose we have:

  int randomNumber(int min, int max);

Equivalentely:

  randomNumber :: Int -> Int -> IO Int

In Haskell if we say

  (+) <$> randomNumber 10 15 <*> randomNumber 10 15

That's the same as

  let x = randomNumber 10 15
  in (+) <$> x <*> x

If we had in C:

  return (randomNumber(10, 15) + randomNumber(10, 15))

That would not be the same as:

  int x = randomNumber(10, 15)
  return (x + x)

Cheers!

-- 
Felipe.