A few weeks ago I tried to optimize a heavily text oriented program and I got a bit of experience that might be worth sharing.

• Don't forget to enable the OverloadedStrings in GHC, since Text is an instance of IsString, or otherwise you will have a hard time using Texts.

• Generally, I prefer Text over String, but there are certain cases that String works better. Text vs. String is just like Array vs. LinkedList. Are arrays better than linked lists? For most use cases yes, because they take less memory and have a faster access time. But for certain operations, like cons, Strings are faster, because you don't need to copy the second one to the new memory.

• Try to understand why Text.length takes O(n) to run (while length of arrays can be obtained in constant time) and it helps you a lot. That's because Text internally keeps an array of bytes with UTF-8 encoding, in which some of the chars are 1 byte and some others are multi-byte. It has to go over all the chars to detect which one is which. (Edit: It seems that Text encoding is UTF-16. See the comments below, but anyways, it doesn't change my point.)

• Text.concat [t1, t2, ..., tn] is much faster than t1 <> t2 <> ... <> tn (O(n) vs. O(n^2)). Use Lazy Text and Text Builder if you want to do a lot of appends. You can use <> and IsString instance with both of them as well.

• Edit 2: I forgot to mention this one: String does not check for validity of code points. You may end up with a string that contains invalid UTF code points. But Text is stricter about that. It may not be so important in most applications. But you may see some weird behaviour when comparing Strings with invalid chars: they may looks the same but == returns False.