Haskell - syntax in do blocks (using IO)

Question

The compiler says

The last statement in a 'do' construct must be an expression:
rmax <- getInteger

when attempting to load a file containing the following snippets of code:

getInteger :: IO Integer
getInteger = readLn

main :: IO ()
main = do  
    putStrLn "specify upper limit of results"  
    rmax <- getInteger
    if rmax `notElem` mot
        then do putStrLn "run again and enter a multiple of 10"
        else do print pAllSorted

What does it (the compiler message) mean, and why does it occur here? (whereas it doesn't in:)

main = do   
    line <- getLine  
    if null line  
        then return ()  
        else do  
            putStrLn $ reverseWords line  
            main  

reverseWords :: String -> String  
reverseWords = unwords . map reverse . words

(above example taken from http://learnyouahaskell.com/input-and-output)

Because the rmax <- rhing is not the last line in your do construct according to what you post, you may have an indentation problem. Make sure you do not have tabulators anywhere. — Ingo, Jan 02 '12 at 11:20
the "stray tab" is clearly seen [here](https://stackoverflow.com/revisions/68375ed1-56e0-45af-935a-81f3f947eac2/view-source). this type of error can be entirely avoided by always using the { ; } explicit separators. then spaces or tabs can't break code, however incoherent the visual appearance might be. — Will Ness, Jul 21 '19 at 14:28

score 7 · Accepted Answer · answered Jan 02 '12 at 11:22

Your indentation is probably messed up because of mixed tabs and spaces. In fact, there appears to be a stray tab in the code snippet in your question, which I'm assuming you pasted directly from your source file.

Most likely, GHC is interpreting the tabs differently from how your editor displays them, so it thinks the do block ends after the line in question.

As a rule of thumb, it's best to use only spaces in Haskell. The language defines very specific rules for interpreting tabs that most code editors don't agree with, but spaces are unambiguous and consistent.

it was... thanks. I have now changed my notepad++ settings to not use tabs. — , Jan 02 '12 at 11:26

Haskell - syntax in do blocks (using IO)

1 Answers1