[Haskell-cafe] isLetter vs. isAlpha

[Artyom Kazak]

Hello!

I saw a question on StackOverflow about the difference between isAlpha and  
isLetter today. One of the answers stated that the two functions are  
interchangeable, even though they are implemented differently.

I decided to find out whether the difference in implementation influences  
performance, and look what I found:

> import Criterion.Main
> import Data.Char
>fTest name f list = bgroup name $ map (\(n,c) -> bench n $ whnf f c) list
>tests = [("latin", 'e'), ("digit", '8'), ("symbol", '…'), ("greek", 'λ')]
>main = defaultMain [fTest "isAlpha" isAlpha tests, 
>                     fTest "isLetter" isLetter tests]

produces this table (times are in nanoseconds):

                  latin digit symbol greek
                  ----- ----- ------ -----
        isAlpha  | 156   212   368    310
        isLetter | 349   344   383    310

isAlpha is twice as fast on latin inputs! Does it mean that isAlpha should  
be preferred? Why isn’t isLetter defined in terms of isAlpha in Data.Char?