Running executable files using Haskell

Question

Say there's a C++ code that I've compiled into an executable using:

g++ test.cpp -o testcpp

I can run this using Terminal (I'm using OS X), and provide an input file for processing inside the C++ program, like:

./testcpp < input.txt

I was wondering if doing this is possible, from within Haskell. I have heard about the readProcess function in System.Process module. But that only allows for running system shell commands.

Doing this:

out <- readProcess "testcpp" [] "test.in"

or:

out <- readProcess "testcpp < test.in" [] ""

or:

out <- readProcess "./testcpp < test.in" [] ""

throws out this error (or something very similar depending on which one of the above I use):

testcpp: readProcess: runInteractiveProcess: exec: does not exist (No such file or directory)

So my question is, whether doing this is possible from Haskell. If so, how and which modules/functions should I use? Thanks.

EDIT

Ok, so as David suggested, I removed the input arguments and tried running it. Doing this worked:

out <- readProcess "./testcpp" [] ""

But I'm still stuck with providing the input.

Answer 1

The documentation for readProcess says:

readProcess
  :: FilePath   Filename of the executable (see RawCommand for details)
  -> [String]   any arguments
  -> String     standard input
  -> IO String  stdout

When it's asking for standard input it's not asking for a file to read the input from, but the actual contents of standard input for the file.

So you'll need to use readFile or the like to get the contents of test.in:

input <- readFile "test.in"
out <- readProcess "./testcpp" [] input