Discovering Haskell Adhoc Polymorphism

This blog is built with Hakyll. Hakyll uses Pandoc for markdown to html conversion. The blog breaks approximately once an year with a change to the underlying pandoc-types.

One of the past breaking changes was the conversion basic text data type from [Char] to Data.Text. The change broke some of the filters I used in the blog. One of the filters was a function to strip the initial / from any resources used in the blog. Hakyll uses URLs with slash at the beginning ‘/’ for local images/resources. I used this filter to fixup the link to images to relative links before converting them into Word document or PDF format.

stripSlash :: String -> String
stripSlash (x:xs)
  | '/' == x = xs
  | otherwise = [x] ++ xs
stripSlash _ = []

I was looking for a way to modify the stripSlash function to support both [Char] and Data.Text data type. Thats how I discovered Haskells ad-hoc polymorphism. This required defining a typeclass and creating an instance of it for each data type I needed to support.

{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE InstanceSigs #-}
import qualified Data.Text as T

class StringUtils a where
  stripSlash :: a -> a

instance StringUtils String where
  stripSlash :: String -> String
  stripSlash (x:xs)
    | '/' == x = xs
    | otherwise = [x] ++ xs
  stripSlash _ = []

instance StringUtils T.Text where
  stripSlash :: T.Text -> T.Text
  stripSlash x = T.pack (stripSlash $ T.unpack x)

Here are a couple of links that helped me.

1. https://stackoverflow.com/questions/12430660/creating-polymorphic-functions-in-haskell
2. https://en.wikibooks.org/wiki/Haskell/Polymorphism#Ad-hoc_polymorphism

I am one of todays lucky 10000. :-)