Rail is an esoteric two-dimensional language created by Jonathon Duerig. This is an interpreter and a C99-backend compiler, written in Haskell by Michael Tolly.

Usage

After cloning the repository, install the Haskell Platform (or just ghc and cabal-install) and install the package with cabal:

cd rail
cabal install

This installs the rail library, and the hrail executable. On Mac/Linux it is placed in ~/.cabal/bin by default, so optionally make sure that is in your $PATH. To run the executable:

hrail input.rail
hrail -c input.rail output.c
hrail -r input.rail output.rail

The first form directly runs the Rail program. The second generates a C program. The third "cleans up" the Rail code so all lines are read horizontally, left-to-right. This can be useful for debugging.

Code generation

The interpreter/compiler starts by statically traversing each function to build a simplified control flow graph. This enables simple C code using goto statements to travel between (more or less) basic blocks. As an example, the following Rail function (a cat program):

$ 'main' (--):
 \
 | /---------\
 | |         |
 | \    /-io-/
 \---e-<
        \-#

becomes this C function:

void fun_main() {
  goto E_5_4;
E_5_4:
  builtin_eof();
  if (pop_bool()) {
    goto done;
  } else {
    builtin_input();
    builtin_output();
    goto E_5_4;
  }
done:
  return;
}

where the only remnant of the grid is a single label (the point where two branching paths meet).