The hpp Executable

hpp is a Haskell pre-processor that is also a C90-compatible pre-processor (with the addition of a --cpp flag). It is packaged as both a library and an executable.

To use as a Haskell preprocessor for resolving #ifdef conditionals and macro expansion, an invocation might look like,

hpp -DDEBUG Foo.hs

To use as a C preprocessor, an invocation might look like,

hpp -DDEBUG --cpp foo.c

To have GHC use hpp as the C pre-processor, add this line to the top of a Haskell source file that makes use of the CPP LANGUAGE pragma,

{-# OPTIONS_GHC -cpp -pgmPhpp #-}

Or add this line to your .cabal file:

ghc-options: -pgmPhpp

Note that you will need to ensure that the hpp executable is available in your build environment (e.g. you can add hpp as a build-depends in your .cabal file).

The hpp Library

The hpp executable is a command-line interface to the hpp library. While the hpp package has been designed to have minimal dependencies beyond what the GHC compiler itself uses, it does include a few small, framework-free unit tests that demonstrate basic usage as a library. In the testIf example, we preprocess the sourceIfdef input with a starting definition equivalent to #define FOO 1. In testArith1, we exercise basic integer arithmetic and comparison. The hppHelper function shows how to run your source input through the preprocessor: expand initialState (preproces mySource). If you want to support #includeing other files, you will use runHpp or streamHpp that operate in IO and support searching among a list of include paths.

{-# LANGUAGE OverloadedStrings #-}
import Control.Monad.Trans.Except
import Data.ByteString.Char8 (ByteString)
import Data.Maybe (fromMaybe)
import Data.Monoid ((<>))
import Hpp
import System.Exit

sourceIfdef :: [ByteString]
sourceIfdef = [ "#ifdef FOO"
              , "x = 42"
              , "#else"
              , "x = 99"
              , "#endif" ]

sourceArith1 :: ByteString -> [ByteString]
sourceArith1 s = [ "#define x 3"
                 , "#if 5 + x > " <> s
                 , "yay"
                 , "#else"
                 , "boo"
                 , "#endif" ]

hppHelper :: HppState -> [ByteString] -> [ByteString] -> IO Bool
hppHelper st src expected =
  case runExcept (expand st (preprocess src)) of
    Left e -> putStrLn ("Error running hpp: " ++ show e) >> return False
    Right (res, _) -> if hppOutput res == expected
                      then return True
                      else do putStr ("Expected "++show expected++", got")
                              print (hppOutput res)
                              return False

testElse :: IO Bool
testElse = hppHelper emptyHppState sourceIfdef ["x = 99\n","\n"]

testIf :: IO Bool
testIf = hppHelper (fromMaybe (error "Preprocessor definition did not parse")
                              (addDefinition "FOO" "1" emptyHppState))
                   ["x = 42\n","\n"]

testArith1 :: IO Bool
testArith1 = (&&) <$> hppHelper emptyHppState (sourceArith1 "7") ["yay\n","\n"]
                  <*> hppHelper emptyHppState (sourceArith1 "8") ["boo\n","\n"]

main :: IO ()
main = do results <- sequenceA [testElse, testIf, testArith1]
          if and results then exitWith ExitSuccess else exitWith (ExitFailure 1)



Added the expand API for pure macro processing (i.e. #includes are ignored).


  • Redesigned library API The Hpp module exports the main pieces. Hpp.Env, Hpp.Types, and Hpp.Config may be used for configuring the preprocessor.


  • Simplify the parsing machinery
  • Don't remove C++-style single-line comments
  • Don't error on unknown cpp directives Previously, a line beginning with "#-}" would cause an error
  • Don't do trigraph replacement by default. Haskell allows "??" in operator names and you can be sure lens uses it!


Address a change wherein GHC 8 will pass -include arguments without a space between "-include" and the file to be included.


Switch to a stream processing model.

This library is designed to have minimal dependencies, so we now have a bespoke implementation of a cross between the pipes and machines libraries included.

This change was done to make some parsing operations easier, believe it or not. For example, most pre-processing is done on a line-by-line basis, but we must also support macro function applications that cross line boundaries. Thus the expansion logic can not merely be given one line at a time from an input file. Previously, a heuristic tried to combine consecutive lines before the parsing stage. Now, the parser itself is able to pull tokens in across lines when necessary.

TL;DR: The upshot is that processing /usr/include/stdio.h on OS X (a surprisingly complicated file!) now uses 78% of the time and 0.38% the memory of previous versions of hpp.


First release!

Used by 3 packages:
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