Leandro Penz wrote:
I have some functions that build big strings by calling other functions and appending the result, like
buildStuff = func1 ++ func2 ++ func3 ++ func4 The usual idiom is something like
buildStuff = concat [func1, func2, func3, func4] Put the list elements on separate lines if it makes life easier.
My idea is to have a monad with a concatenating >>, so that I can:
bulidStuff = do func1 func2 func3 func4
I am thinking about using the writer monad, but I would still have to use "tell". The list monad is not an option either, as it does not concatenate.
Is there a proper way of doing something like this (efficiently) ? The writer monad is a more complicated way of doing this, and might be appropriate if you want to build up a string over a lot of different functions. But its probably easier to use concat.
One thing: beware of the copying overhead. The naive use of the Writer monad is to use ++, but every time you "tell" it a new string the accumulated string so far gets copied to have the new one appended. The way around this is to use StringS (which you will find in the Prelude). StringS has the type "String -> String", which seems really wierd. An StringS takes a string, prepends something on to it, and then returns the resulting string. The clever thing is that you can take two StringS and concatenate them using (.) function composition in constant time. So I can write: foo, bar, stop, foobar :: StringS foo = ("Hello " ++) -- A function from a string to a string. bar = ("world" ++) stop = ('.' :) -- Also a function from a string to a string: work it out. foobar = foo . bar . stop -- Function composition: very efficient. main = do putStrLn (foobar "") -- Get the final result by applying the function to an empty string. You can do this inside the Writer monad as well because functions, like strings, are instances of the Monoid class (i.e. they implement mplus in the way you would expect). You just have to wrap a function around "tell" like say str = tell (str ++) and remember that runWriter will then hand you a StringS. Hope this helps, Paul.