Haskell — “The last statement in a 'do' construct must be an expression”

问题

Like it says in the title: What does The last statement in a 'do' construct must be an expression mean? I ended my do block with a putStrLn like it shows in several examples I've seen, and i get an error.

Code:

main = do args <- getArgs
           file <-readFile "TWL06.txt"
           putStrLn results

回答1:

Most of the time, it's because your code is mis-aligned and compiler assumes that your "do" block ended prematurely (or has extra code that dont really belong there)

回答2:

Your last line isn't something like someVar <- putStrLn "hello", by any chance, is it? You'll get that error if you try to do a variable binding on the last line, because it's equivalent to putStrLn "Hello" >>= \someVar -> — it expects there to be an expression at the end.

回答3:

Incorrect indentation can lead to this error. Also, is good not to use tabs, only spaces.

来源：https://stackoverflow.com/questions/2016963/haskell-the-last-statement-in-a-do-construct-must-be-an-expression