cabal build gives parse error, but code works fine in GHCi

Question

My code works in GHCi, but when I try to build the project with cabal build, it gives the error parse error on input ‘<-’

Here's a minimal example:

foo :: IO ()
foo = do
    let x = do
        b <- getLine
        return b
    return ()

Answer 1

It turns out that my GHCi is set up to use the -XNondecreasingIndentation extension, which can be seen with the command :show language

base language is: Haskell2010
with the following modifiers:
  -XNoDatatypeContexts
  -XNondecreasingIndentation

Without this extension this is bad syntax:

foo :: IO ()
foo = do
    let x = do
        b <- getLine
        return b
    return ()

but this is okay:

foo :: IO ()
foo = do
    let 
        x = do
            b <- getLine
            return b
    return ()

To fix the problem, add

  default-extensions: 
    NondecreasingIndentation

to the .cabal file, or if you prefer add {-# language NondecreasingIndentation #-} to just this one module. Alternatively, reformat the code as above, or use Haskell98 instead of Haskell2010.