CODEWITHSUNDEEP
haskelloverriding
peterxz
peterxz

Reputation: 874

Overriding functions for different data types

Im trying to define a function which I want to behave slightly differently depending on what datatype is passed in as the argument, strings and integers. How would one go about doing this? Or are there any changes you could suggest to somehow compose these two into one function. Ive read that trying to validate the datatype is not very haskell like so I thought overriding might be the way to go to do this but the compiler doesnt like the duplicate :( 

jumpTo :: Int -> [String] -> [String]
jumpTo index t = do 
    let (x,y) = splitAt index t
    init x ++ [last x ++ "QQ"] ++ y

jumpTo :: String -> [String] -> [String]
jumpTo string t = do 
    pos <- [fromJust (findWord string t)]
    let (x,y) = splitAt pos a
    init x ++ [last x ++ "QQ"] ++ y

In words, I want a function jumpTo which can either be passed a String or Int as its first argument; if its a string, I find it in the list, and for an integer I do the operations with the index straight away. It just seems more elegant to have the same function doing this.

Upvotes: 1

Views: 302

Answers (1)

Fyodor Soikin
Fyodor Soikin

Reputation: 80744

This is exactly the idea behind type classes: you define your function (or a group of functions) not by itself, but as belonging to a type class, which is tagged with a type variable (or several). Then you define one or more instances of the type class with different types substituted for that type variable, and you define separate bodies for the function in each instance. The end effect is that the function has different body depending on the types of its arguments or result.

In your example:

class JumpTo a where
    jumpTo :: a -> [String] -> [String]

instance JumpTo Int where
    jumpTo index t = do 
        let (x,y) = splitAt index t
        init x ++ [last x ++ "QQ"] ++ y

instance JumpTo String where
    jumpTo string t = do 
        pos <- [fromJust (findWord string t)]
        let (x,y) = splitAt pos a
        init x ++ [last x ++ "QQ"] ++ y

Upvotes: 5

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