[Haskell-beginners] Equivalent of IO Monad in other functional languages?

[Sumit Sahrawat, Maths & Computing, IIT (BHU)]

Frerich, the function would still be pure, even if it returns a poisoned
recipe as you say. Thus, a little thought leads me to believe that the
pattern would still not break if no unsafe* functions are used.


On 16 March 2015 at 20:28, Sumit Sahrawat, Maths & Computing, IIT (BHU) <
sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:

> Sorry for having strayed towards a different path. I'll try to answer only
> your question this time :)
>
> An example from [1], for SML (bottom of the page) shows a function, with
> the following type signature in haskell
>
>     copyTextFile :: String -> String
>
> One cannot write such a function in haskell that does the same thing
> because the example uses IO within a function with a pure looking type.
>
> I'm sure people experienced with other functional languages will have good
> examples, so I'll leave it to them.
>
> To answer your second question, I'm still not sure :).
> I was just relaying how I understood purity, so that somebody might also
> correct me if I'm wrong.
>
> Summarizing the discussion above, a function is pure if it gives the same
> output for the same input. Now a function returning 'IO a' always returns
> the same IO instructions (these instructions might be impure), but the
> function (by the virtue of always returning the same output for the same
> inputs) is ultimately pure. Frerich has also raised an important point that
> an IO computation might actually depend on some random file, which breaks
> this pattern.
>
> [1]: http://www.cs.cornell.edu/courses/cs312/2006fa/recitations/rec09.html
>
> On 16 March 2015 at 20:15, Simon Kitching <simon at vonos.net> wrote:
>
>> On 03/16/2015 03:04 PM, Heinrich Apfelmus wrote:
>>
>>> Sumit Sahrawat, Maths & Computing, IIT (BHU) wrote:
>>>
>>>> On 15 March 2015 at 23:19, <amindfv at gmail.com> wrote:
>>>>
>>>>  From that perspective isn't every language pure? Haskell's still got
>>>>> "randomIO" and "print <=< readMVar"
>>>>>
>>>>
>>>> Haskell is impure only when you use the unsafe* functions (atleast
>>>> that's
>>>> how I understand purity).
>>>>
>>>> My understanding is that a language is impure if it allows one to write
>>>> arbitrary IO within a function and still give it a proper (mathematical)
>>>> function type. In other words impurity arises only if you can unwrap
>>>> the IO
>>>> monad (which is what the unsafe functions do).
>>>>
>>>> The two examples you give above are pure under such a perspective, but I
>>>> might be wrong.
>>>>
>>>
>>> You're right, indeed. A language is pure if supplying a value `x` of
>>> type `A` to the a function
>>>
>>>     A -> B
>>>
>>> will always returns the same result, no matter how often or in which
>>> order this function is called. This is true for both
>>>
>>>     randomIO             :: Random a => IO a
>>>     (readMVar >=> print) :: Show a   => MVar a -> IO ()
>>>
>>> because they return an IO action. This action will always be the same
>>> given the same arguments.
>>>
>>> The language would be impure if these functions had the types
>>>
>>>     randomIO             :: Random a => a
>>>     (readMVar >=> print) :: Show a   => MVar a -> ()
>>>
>>
>> Doesn't "pure" correspond to "can write a unit test for"? When a
>> function's return value only ever depends on its inputs, then I can
>> write a set of test-cases for various inputs, and assert that the
>> return-value has the expected content.
>>
>> A function that reads from a file, stdin, etc. cannot be tested in the
>> same way; I cannot _assert_ that the returned value has specific content.
>>
>>  Sumit stated earlier that IO is "a pure program that can be executed",
>> which seems similar to your description above (which I admit I
>> don't yet 100% understand). How can I "assert" anything about this
>> "program", or make a Haskell-based application 'safer' in any way by
>> leveraging this kind of 'purity'?
>>
>> Thanks,
>> Simon
>>
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>
>
>
> --
> Regards
>
> Sumit Sahrawat
>



-- 
Regards

Sumit Sahrawat
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