Hello KwangYul, Supporting this sort of parametricity is one of the goals of the Backpack project. You can see some code that demonstrates how this might be done: https://github.com/yihuang/tagstream-conduit/pull/18 Maybe you'll get to use this feature in GHC 8.2! A big problem with StringLike type classes is that you must pre-commit to the set of operations which are supported before hand. For strings, there are many, many such operations, and it is difficult to agree a priori what these operations should be. Edward Excerpts from KwangYul Seo's message of 2016-07-13 17:40:13 -0700:
Hi all,
There are multiple string types in Haskell – String, lazy/strict ByteString, lazy/strict Text to name a few. So to make a string handling function maximally reusable, it needs to support multiple string types.
One approach used by TagSoup library is to make a type class StringLike which represents the polymorphic string type and uses it where a String type is normally needed. For example,
parseTags :: StringLike str => str -> [Tag str]
Here parseTags takes a StringLike type instead of a fixed string type. Users of TagSoup can pick any of String, lazy/strict ByteString, lazy/strict Text because they are all instances of StringLike type class.
It seems StringLike type class is quite generic but it is used only in the TagSoup package. This makes me wonder what is the idiomatic way to support multiple string types in Haskell. What other approaches do we have?
Thanks, Kwang Yul Seo