[Haskell-cafe] Re: Function to cast types

[Achim Schneider]

Anonymous Anonymous <temp.public at gmail.com> wrote:

> Hello,
> 
> I'm new to haskell, I'm wondering how can you write a function that
> will do the following:
> 
> fromIntToString :: Int -> String
> 
> this is a cast function to cast an Int to a String. I know such
> function exist, however let's assume it didn't exist. How would I
> write such function? 
>
I have no Idea, but this one works:

intOfChar c | c >= '0' && c <= '9' = fromEnum c - fromEnum '0'
intOfChar _ = undefined

intOfString = sum . map (uncurry (*)) . zip (map (10^) [0..]) 
            . map intOfChar . reverse

...assuming that 0..9 are consecutive in the Char Enum, which they are.

Reading intOfString backwards, you get: reverse the string, change each
digit to its integer value, pair it up with the right power of 10,
multiply those pairs, and finally sum everything up.

You can replace map f . zip with zipWith f , but that doesn't change
much.

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