Forced strictness for lists in haskell

Question

I made really time consuming algorithm which produces a short string as the result. When I try to print it (via putStrLn) it appears on the screen character by character. I did understand why that happened, and I tried to force evaluation of the string before actual printing.

myPrint !str = putStrLn str

But this help very little. When I ran the program in debug I noticed that the !str forced evaluation only for the first character.

Does anyone know why is that, and how to deal with this?

Answer 1

(!) translates into seq, which evaluates strictly to Weak Head Normal Form -- that is, it only evaluates to the outermost constructor. To evaluate more deeply, you need a "deep" form of seq.

This is known as deepseq.

It is in the deepseq package.

Answer 2

seqList :: [a] -> ()
seqList [] = ()
seqList (x:xs) = strictList xs