Handling error in function in Haskell

Question

I have a sign function, which can return an error.

signe :: Int -> Char
signe chiffre
    | chiffre >= 1 && chiffre <= 9 = '+'
    | chiffre == 0 = '0'
    | chiffre >= -9 && chiffre <= (-1) = '-'
    | otherwise = error "Erreur in the sign"

I'd like to make a simple one to return the corresponding code of the sign, but with error handling.

signes liste = [ signe x | x<-liste ]

I give you an example : For now, if I call

signes [1,3,0,-10]

it gives me

++0*** Exception: Error in sign.

I'd like to have nothing instead of Exception: ++0.

I wanted to start my message by "hi everyone" but even with editing it's not taking in account ... — maxime1992
– maxime1992, Commented Nov 14, 2013 at 17:57

Ingo · Accepted Answer · 2013-11-14 18:18:53Z

3

You can, and should, use Maybe in such cases:

signe chiffre 
   | chiffre >= 1 && chiffre <= 9 = Just '+'
   ....
   | otherwise = Nothing    -- parbleu!!

signes = mapMaybe signe

You may need to import Data.Maybe for the mapMaybe function.

edited Nov 14, 2013 at 18:18

answered Nov 14, 2013 at 18:00

Ingo

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6 Comments

maxime1992 Over a year ago

mapMaybes --> "Not in scope". (I've added at the top of my sheet "import Data.List") So i just tryed signes = map signe But i get [Just '+' , ........, Nothing ]

Alex Reinking Over a year ago

It's actually just mapMaybe. See the docs.

Ingo Over a year ago

Thank you for pointing it out, @AlexReinking , it is actually mapMaybe and it is in Data.Maybe. Always confuse that with catMaybes where there is an s at the end. COrrected above.

maxime1992 Over a year ago

I had a look on the docs and i tried mapMaybe in case too, before answering. Same problem ... Here are my functions : import Data.List signe chiffre | chiffre >= 1 && chiffre <= 9 = Just '+' | chiffre == 0 = Just '0' | chiffre >= -9 && chiffre <= (-1) = Just '-' | otherwise = Nothing signes = mapMaybe signe And i'm sorry to write here but "Users with less than 10 reputation can't answer their own question for 8 hours after asking. You can answer 11/15/2013 1:56:02 AM. Until then please use comments, or edit your question instead." ...

maxime1992 Over a year ago

Ingo thanks it worked !! Thanks to all of you :) !! Another Haskell question is : Would it be possible to create a SIMPLE code to know if a list is increasing ? Maybe with foldr or another thing but to make something simple ?

|

bheklilr · Accepted Answer · 2013-11-14 18:00:47Z

2

A better way would be to actually use the Maybe type which lets you literally return Nothing or Just aValue. You could rewrite your function as

signe :: Int -> Maybe Char
signe chiffre
    | chiffre >= 1 && chiffre <= 9 = Just '+'
    | chiffre == 0 = Just '0'
    | chiffre >= (-9) && chiffre <= (-1) = Just '-'
    | otherwise = Nothing

answered Nov 14, 2013 at 18:00

bheklilr

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Comments

jamshidh · Accepted Answer · 2013-11-15 18:00:28Z

0

The problem already seems to be answered by Ingo, but I wanted to point out that since you had an error message in the original question, perhaps "Either" would be a better choice here

signe :: Int -> Either String Char
signe chiffre
    | chiffre >= 1 && chiffre <= 9 = Right'+'
    | chiffre == 0 = Right '0'
    | chiffre >= -9 && chiffre <= (-1) = Right '-'
    | otherwise = Left "Erreur in the sign"

where you can get the filtered list with

signes liste = [ x | Right x<-map signe liste ]

Both Maybe and Either are used for error checking, Either gives you the ability to pass an "Exception" up the call chain.

answered Nov 15, 2013 at 18:00

jamshidh

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