[Haskell-beginners] Case vs Guards. I still don't know what is the difference

[Semih Masat]

Sorry, if i am flooding.

To make it clear what i wanted to say in last section on previous mail.



Lets say i have a list of numbers and i want to do different things in case
of different even numbers on that list.

If i use guards i will do it like this :

nums = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]
howManyEvens = length(removeOdd(nums))

isItOk count
    | count > 10 = "Too much"
    | count > 8   = "Isn't this a little much?"
    | count > 5   = "I think this is ok"
    | count > 3   = "Little more please"
    | count > 0   = "Ooo, cmon"
    | otherwise   = "We gonna die"

result = isItOk howManyEvens

This is a very stupid example but this will work i guess.

And if i wanted to this with *case* , i will do it like;

isItOk' nums = case (length(removeOdd(nums))) of
    10  -> "Too much"
    8   -> "Isn't this a little much?"
    5   -> "I think this is ok"
    3   -> "Little more please"
    0   -> "Ooo, cmon"
    x   -> "i don't even"

So the only different thing is i didn't need to create *howManyEvens*
constant.


PS: While i writing this. I realized that with case, i need to use pattern
matching but with guards i can use other functions if i wanted to. ( like
count > 10 )
Sorry for asking prematurely. And if anyone reaches this email by google
search. Look at this explanation : http://stackoverflow.com/a/4156831

To the authors : Please, if you writing a book a blog post about haskell.
Don't create same function in different styles. We don't understand which
one we need to use and why we have all different choices.

Thanks.
Semih

On Thu, Jul 7, 2016 at 3:43 AM, Semih Masat <masat.semih at gmail.com> wrote:

> Hello,
>
> I am new to Haskell and trying to learn it with learnyouahaskell.com and
> Pluralsight Haskell course.
>
> And i have a very noob question.
>
> I understand that *if .. else* is just a syntactic sugar over *case. *But
> what about guards then ?
>
> Are guards also *case *in different syntax ? Or vice versa ? Like with an
> example.
>
>
> anyEven nums
>     | (length (removeOdd nums)) > 0 = True
>     | otherwise                     = False
>
>
> anyEven' nums = case (removeOdd nums) of
>     []        -> False
>     (x:xs)  -> True
>
> I can do the same thing with both of them.
>
> As i understand the only different thing is, with *case *i can manipulate
> the parameter (like here in the example i used removeOdd) and can use the
> manipulated parameter to decide what to do after that.
> So i will not need to use removeOdd function inside the case. ( maybe i
> will need to use in every guard definition if i choose to use guards )
>
> Is this it?
>
> Is this the only difference between them ?
>
> And if it is, why haskell needed do implement both of them. Can't we use
> function like removeOdd before using it on case or guard functions ?
>
>
> Thanks, and sorry if my english is bad.
>
> Semih Masat
>
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