Haskell 中 forM 的返回值

Question

If I have a list input = ["1", "-2", "14", "3"] with length n and I want to convert it into an Int list, what is the return value of

forM [0..(n-1)] $ \i -> do
        let t = read (input!!(i)) :: Int 
        return ()

I figured out that t will contain the Int value of each string from the input list but I don't understand how should I put them in a new list. Note that this forM syntax is not written by me. I researched how I should work with monads but I still don't really get it since I couldn't find a single example.

Answer 1

If I have a list input = ["1", "-2", "14", "3"] with length n and I want to convert it into an Int list.

Then you should use:

 input :: [Int]

Indeed:

Prelude> input = ["1", "-2", "14", "3"]
Prelude> map read input :: [Int]
[1,-2,14,3]

I figured out that t will contain the Int value of each string from the input list but I don't understand how should I put them in a new list.

Variables in (pure) functional languages, like Haskell, are immutable . That means that once a variable is set to a value, you can not set it to a different value.

In fact in Haskell, one seldomly use looping mechanisms like forM , etc. The M suffix usually says you are working with M onads, but here you do not need monads at all. I strongly advice not to use do , etc. before you have some basic understanding on how a monad works. See for example the A fistful of Monads chapter on Learn You A Haskell [lyah] .