Get sqrt from Int in Haskell

Question

How can I get sqrt from Int.

I try so:

sqrt . fromInteger x

But get error with types compatibility.

Mariken

Answer 1

Using fromIntegral:

Prelude> let x = 5::Int
Prelude> sqrt (fromIntegral  x)
2.23606797749979

both Int and Integer are instances of Integral:

• fromIntegral :: (Integral a, Num b) => a -> b takes your Int (which is an instance of Integral) and "makes" it a Num.

• sqrt :: (Floating a) => a -> a expects a Floating, and Floating inherit from Fractional, which inherits from Num, so you can safely pass to sqrt the result of fromIntegral

I think that the classes diagram in Haskell Wikibook is quite useful in this cases.

Chlorophyll

Could sqrt (fromIntegral x) also be written as sqrt $ fromIntegral x? – Cetacean

Yes indeed it could, since explicit application ($) does not bind as tightly as implicit application. – Labored

or even sqrt . fromIntegral $ x – Milord

I prefer to write it as ((((($)))((sqrt))(fromIntegral (x)))). – Alister

Answer 2

Perhaps you want the result to be an Int as well?

isqrt :: Int -> Int
isqrt = floor . sqrt . fromIntegral

You may want to replace floor with ceiling or round. (BTW, this function has a more general type than the one I gave.)

Shillelagh

ghc warns about this, since it doesn't know which exact type to use between fromIntegral and floor (could be Double,Float, etc.). To fix: isqrt x = floor . sqrt $ (fromIntegral x :: Float), which is less elegant :( – Afeard

I like it, anyway. No warns on GHC 8. – Demigod

ghc will warn if -Wtype-defaults, -Werror=type-defaults. – Illegitimate

Answer 3

Remember, application binds more tightly than any other operator. That includes composition. What you want is

sqrt $ fromIntegral x

Then

fromIntegral x 

will be evaluated first, because implicit application (space) binds more tightly than explicit application ($).

Alternately, if you want to see how composition would work:

(sqrt .  fromIntegral) x

Parentheses make sure that the composition operator is evaluated first, and then the resulting function is the left side of the application.

Labored