DoAndIfThenElse language extension

Have you ever encountered “Unexpected semi-colons in conditional” errors while building your project with Cabal and wondered why? This blog post explains about this puzzling error.

Here is a simple program which prints the given command line arguments.

import System.Environment
 
main = do
    args <- getArgs
    if length args < 1 then
        putStrLn "Usage: DoAndIfThenElse [args]"
    else
        putStrLn $ concat args

You can build this Haskell program with ghc –make:

ghc --make DoAndIfThenElse.hs
[1 of 1] Compiling Main             ( DoAndIfThenElse.hs, DoAndIfThenElse.o )
Linking DoAndIfThenElse ...

Okay. Then let’s create a Cabal build script and build this program with Cabal.

name:            DoAndIfThenElse
version:         0.0.1
cabal-version:   >= 1.8
build-type:      Simple
 
executable DoAndIfThenElse
  hs-source-dirs:    src
  main-is:           DoAndIfThenElse.hs
  build-depends:     base

The source code is exactly the same, but now GHC suddenly complains about the unexpected semi-colons we never inserted anyway.

src/DoAndIfThenElse.hs:5:8:
    Unexpected semi-colons in conditional:
        if length args < 1 then putStrLn
                                  "Usage: DoAndIfThenElse [args]"; else putStrLn $ concat args
    Perhaps you meant to use -XDoAndIfThenElse?

You can fix this problem by adding DoAndIfThenElse language pragma at the beginning:

{-# LANGUAGE DoAndIfThenElse #-}

Or you can fix it by changing the indentation of then and else of if expression.

if length args < 1
    then putStrLn "Usage: DoAndIfThenElse [args]"
    else putStrLn $ concat args

So the problem is on the indentation. You have to keep then and else at deeper indentation levels than the if block they belong.

ghc –make is okay because GHC automatically turns on the syntax extension DoAndIfThenElse. However, Cabal is more picky, so you have to turn it on manually either at the top of your code files, or in your Cabal files.

There is also a StackOverflow question on this issue, Unexpected semi-colons in conditional.