CODEWITHSUNDEEP
haskell
Yitzak Hernandez
Yitzak Hernandez

Reputation: 357

Haskell how to add a float to all members of an integer list

I'm new to Haskell, but I'm trying to add a float to all members of an integer list in Haskell and I'm having trouble getting past the errors and such.

Basically

addAll xs = map (+3.5) xs

Where xs is an integer list.

Upvotes: 0

Views: 953

Answers (2)

epsilonhalbe
epsilonhalbe

Reputation: 15967

What would you expect the result list to be?

It can be [Float] or [Integer] - in Haskell you should be explicit about that.

1. [Integer]

You need either to round the floating point number before you multiply:

map (+ (round (3.5 :: Float))) [1..(10 :: Integer)]

2. [Float]

Or convert the integers with fromInteger

[3.5 + fromInteger x | x <- xs]
map ((+3.5) . fromInteger) xs

as adding numbers only works if both summands are of the same type, you can see this in the type signature

(+) :: Num a => a -> a -> a

all as have to be the same and be an instance of the Num-typeclass (much alike an interface in Java, if you have experience with that).

Note: There is no automatic conversion in Haskell1.

1: Except for numeric literals like 0,1,2… they are converted using fromInteger, numeric literals like 1.1,2.2 etc. are converted by fromRational and String literals when you enable OverloadedStrings with fromString.

Upvotes: 5

Chris Martin
Chris Martin

Reputation: 30736

You'll need to convert each Integer to a Float before you can add a Float to it.

You can use fromInteger for this. Its type is

Num a => Integer -> a

which, for our purpose here, specializes to

Integer -> Float

So here's what the addAll function looks like:

addAll :: [Integer] -> [Float]
addAll xs = map (\x -> fromInteger x + 3.5) xs

Example usage in GHCi:

λ> addAll [1, 2, 3]
[4.5,5.5,6.5]

Upvotes: 7

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