[Haskell-cafe] Re: Functional version of this OO snippet

[Thomas Davie]

On 5 Dec 2008, at 16:42, Apfelmus, Heinrich wrote:

> Thomas Davie wrote:
>>> You don't even need a type class, a simple data type is enough.
>>
>> Very true, but I disagree that you've made it functional in any  
>> way, IO
>> is all about sequencing things, it's very much not a functional style
>>>
>>> data Engine = Engine { foo :: IO (), bar :: String -> IO () }
>>
>> This is much nicer done as functions from String -> String
>
> Sure, I agree. I was just replicating  foo  and  bar  from the OP
> because I don't know what kind of effect he had in mind. I mean,  
> instead
> of merely mapping each command in isolation, he could want to  
> accumulate
> a value or read files or something.

Sure, and he could then use a fold instead of a map.  Reading files is  
problematic, but as long as you're only doing it once (the most common  
situation) is entirely fine wrapped up in an unsafePerformIO.

Either way, the question was how to do it functionally, and to do it  
functionally, and with all of the nice shiny benefits we get with  
functional code like composibility and orthogonality, you need to do  
it with String -> String.

Bob