Efficient String Implementation in Haskell

Question

I\'m currently teaching myself Haskell, and I\'m wondering what the best practices are when working with strings in Haskell.

The default string implementation in Has

Answer 1

Best practices for working with strings performantly in Haskell are basically: Use Data.ByteString/Data.ByteString.Lazy.

http://hackage.haskell.org/packages/archive/bytestring/latest/doc/html/

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As far as the efficiency of the default string implementation goes in Haskell, it's not. Each Char represents a Unicode codepoint which means it needs at least 21bits per Char.

Since a String is just [Char], that is a linked list of Char, it means Strings have poor locality of reference, and again means that Strings are fairly large in memory, at a minimum it's N * (21bits + Mbits) where N is the length of the string and M is the size of a pointer (32, 64, what have you) and unlike many other places where Haskell uses lists where other languages might use different structures (I'm thinking specifically of control flow here), Strings are much less likely to be able to be optimized to loops, etc. by the compiler.

And while a Char corresponds to a codepoint, the Haskell 98 report doesn't specify anything about the encoding used when doing file IO, not even a default much less a way to change it. In practice GHC provides an extensions to do e.g. binary IO, but you're going off the reservation at that point anyway.

Even with operations like prepending to front of the string it's unlikely that a String will beat a ByteString in practice.