haskell if else statement with multiple conditions

Question

the goal is to Write a function that, given an integer between 1 and 12 evaluates to the name of the related month (1->January, etc.) Else, it may evaluate to "Incorrect month number". This is my code:

list = ["Jan","Feb","Mar","April", "May", "Jun","Jul","Aug","Sep","Oct","Nov","Dec"]
month x = if x<13 && x>0
          then x = list !! (x-1)
          else "Not a month"

this is the error msg: parse error on input "="

Many thanks for your help

@YassineBotabia: changing it to || would in fact change the semantics and generate errors. Furthemore the error is not with the x = on the first line, but the one on the second line. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Jan 21, 2017 at 11:46

chi · Accepted Answer · 2017-01-21 11:44:53Z

4

Just remove x =. (Why did you write that?)

list = ["Jan","Feb","Mar","April","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"]
month x = if x<13 && x>0
          then list !! (x-1)
          else "Not a month"

answered Jan 21, 2017 at 11:44

chi

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1 Comment

Yassine Boutabia Over a year ago

Thanks you so much for your help.

Babra Cunningham · Accepted Answer · 2017-01-21 12:14:29Z

3

Using guards is the best strategy here:

month x | x > 0 && x < 13      = list !! (x - 1)
        |otherwise             = "Error"

You could also use Maybe for this type of operation:

mylist = ["Jan","Feb","Mar","April", "May", "Jun","Jul","Aug","Sep","Oct","Nov","Dec"]

exampleFunc :: [String] -> Int -> Maybe String
exampleFunc li int
   | int > 0 && int < 13    = Just (li !! (int - 1))
   | otherwise              = Nothing

Using Maybe allows you to return a standard value (Nothing) in the event of an otherwise condition.

edited Jan 21, 2017 at 12:14

answered Jan 21, 2017 at 11:59

Babra Cunningham

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2 Comments

Yassine Boutabia Over a year ago

Thank you Barbra, you comments are very helpful .

chepner Over a year ago

Maybe String should absolutely be used here, since "Not a month" is not, in fact, a month.

willeM_ Van Onsem · Accepted Answer · 2017-01-21 12:42:20Z

2

Well you write:

month x = if x0
          then x = list !! (x-1)
          else "Not a month"

You probably are used to the imperative world (C++, Java,...) where there are variables that you can set and set to other values. In a functional and by extent declarative language, a variable is only set once.

Furthermore there is no need to set x to a value, you only have to return list!!(x-1) so:

month x = if x<13 && x>0
          then list !! (x-1)
          else "Not a month"

should work.

An - in my opinion - more elegant way to write this however is using guards:

month x | x > 0 && x < 13 = list !! (x-1)
        | otherwise = "not a month"

edited Jan 21, 2017 at 12:42

answered Jan 21, 2017 at 11:45

willeM_ Van Onsem

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1 Comment

Yassine Boutabia Over a year ago

Thank you so much. x= was my error. and you are right I just started functional programming.

ƛƛƛ · Accepted Answer · 2017-01-21 15:55:30Z

1

You can also write it like this:

month = ["Jan","Feb","Mar","April", "May", "Jun","Jul","Aug","Sep","Oct","Nov","Dec"]
getMonth k = lookup k $ zip [1..] month

answered Jan 21, 2017 at 15:55

ƛƛƛ

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