nint — Nix INTerpreter

nint is a shebang compatible interpreter for nix. It is currently implemented as a fairly trivial wrapper around nix-instantiate --eval. It allows to run nix expressions as command line tools if they conform to the following calling convention:

• Every nix script needs to evaluate to a function which takes an attribute set as its single argument. Ideally a set pattern with an ellipsis should be used. By default nint passes the following arguments:

  • currentDir: the current working directory as a nix path
  • argv: a list of arguments to the invokation including the program name at builtins.head argv.
  • Extra arguments can be manually passed as described below.

• The return value must either be

  • A string which is rendered to stdout.

  • An attribute set with the following optional attributes:

    • stdout: A string that's rendered to stdout
    • stderr: A string that's rendered to stderr
    • exit: A number which is used as an exit code. If missing, nint always exits with 0 (or equivalent).

Usage

nint [ --arg ARG VALUE … ] script.nix [ ARGS … ]

Instead of --arg, --argstr can also be used. They both work like the flags of the same name for nix-instantiate and may be specified any number of times as long as they are passed before the nix expression to run.

Below is a shebang which also passes depot as an argument (note the usage of env -S to get around the shebang limitation to two arguments).

#!/usr/bin/env -S nint --arg depot /path/to/depot

Limitations

• No side effects except for writing to stdout.

• Output is not streaming, i. e. even if the output is incrementally calculated, nothing will be printed until the full output is available. With plain nix strings we can't do better anyways.

• Limited error handling for the script, no way to set the exit code etc.

Some of these limitations may be possible to address in the future by using an alternative nix interpreter and a more elaborate calling convention.

Example

Below is a (very simple) implementation of a ls(1)-like program in nix:

#!/usr/bin/env nint
{ currentDir, argv, ... }:

let
  lib = import <nixpkgs/lib>;

  dirs =
    let
      args = builtins.tail argv;
    in
      if args == []
      then [ currentDir ]
      else args;

  makeAbsolute = p:
    if builtins.isPath p
    then p
    else if builtins.match "^/.*" p != null
    then p
    else "${toString currentDir}/${p}";
in

  lib.concatStringsSep "\n"
    (lib.flatten
      (builtins.map
        (d: (builtins.attrNames (builtins.readDir (makeAbsolute d))))
        dirs)) + "\n"