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

[Theodore Lief Gannon]

One important feature of guards is that passing a pattern but failing all
the guards falls through to the next pattern. Toy example:

maybeGT5 x | x > 5 = Just x
maybeGT5 _ = Nothing
On Jul 6, 2016 6:17 PM, "Semih Masat" <masat.semih at gmail.com> wrote:

> 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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