Haskell -- "The last statement in a 'do' construct must be an expression"

Question

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

check your indentation. make sure all the lines in the block have the same indentation. tabs and spaces are not the same — newacct, Jan 06 '10 at 23:24
If you're working with indent-sensitive languages like Haskell and Python, do make sure that your editor has sane tab settings. `:set et ts=8 sw=4 sta` or so in Vim. — ephemient, Jan 07 '10 at 00:04

score 29 · Accepted Answer · answered Jan 06 '10 at 23:43

29

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)

answered Jan 06 '10 at 23:43

ADEpt

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Saved the day! Thanks! – hbobenicio Jun 30 '17 at 16:38

score 20 · Answer 2 · answered Jan 06 '10 at 22:57

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.

score 7 · Answer 3 · answered Jan 07 '10 at 00:25

7

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

answered Jan 07 '10 at 00:25

Hai

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Haskell -- "The last statement in a 'do' construct must be an expression"

3 Answers3

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