When I tried to compile a Haskell file , error:parse error on input '#'

Question

I was learning how to use this SDK from https://github.com/spaceships/garbled-circuits

When I finished downloading the dependencies and tried to compile one example, the error was like parse error on input '#' Here is the source code where the error came from:

import Prelude hiding (traverse)
#ifdef DEBUG
import Debug.Trace
#else
trace :: String -> a -> a
trace = flip const
traceM :: Monad m => String -> m ()
traceM _ = return ()
#endif

I guess it was because the C grammar like '#ifdef' was used in a Haskell file that caused this error, but I'd already used cabal install cipher-aes128 to download this SDK's dependencies (maybe).

Answer 1

Referring to the Cabal documentation,

extensions: CPP

should be added to the package description in order to enable the C pre-processor.

Source

Additionally, the language pragma:

{-# LANGUAGE CPP #-}

Also enables the language extension.