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# Helper function to synthesize a directory of "lazy-built" binaries
# that can be added to $PATH inside of a repository.
#
# Using this, a Nix shell environment in some repository can contain
# several slow-to-build commands without blowing up evaluation and
# build time whenever the shell is loaded.
#
# Note that the generated script is deliberately impure to speed up
# evaluation, and expects both `git` and `nix-build` to exist in the
# user's $PATH. If required, this can be done in the shell
# configuration invoking this function.
{ pkgs, ... }:
let
inherit (builtins) attrNames attrValues mapAttrs;
inherit (pkgs.lib) concatStringsSep;
# Create the case statement for a command invocations, optionally
# overriding the `TARGET_TOOL` variable.
invoke = name: { attr, cmd ? null }: ''
${name})
attr="${attr}"
${if cmd != null then "TARGET_TOOL=\"${cmd}\"\n;;" else ";;"}
'';
# Create command to symlink to the dispatch script for each tool.
link = name: "ln -s $target $out/bin/${name}";
invocations = tools: concatStringsSep "\n" (attrValues (mapAttrs invoke tools));
in
# Attribute set of tools that should be lazily-added to the $PATH.
# The name of each attribute is used as the command name (on $PATH).
# It must contain the keys 'attr' (containing the Nix attribute path
# to the tool's derivation from the top-level), and may optionally
# contain the key 'cmd' to override the name of the binary inside the
# derivation.
tools:
pkgs.writeTextFile {
name = "lazy-dispatch";
executable = true;
destination = "/bin/__dispatch";
text = ''
#!${pkgs.runtimeShell}
set -ue
if ! type git>/dev/null || ! type nix-build>/dev/null; then
echo "The 'git' and 'nix-build' commands must be available." >&2
exit 127
fi
readonly REPO_ROOT=$(git rev-parse --show-toplevel)
TARGET_TOOL=$(basename "$0")
case "''${TARGET_TOOL}" in
${invocations tools}
*)
echo "''${TARGET_TOOL} is currently not installed in this repository." >&2
exit 127
;;
esac
result=$(nix-build --no-out-link --attr "''${attr}" "''${REPO_ROOT}")
PATH="''${result}/bin:$PATH"
exec "''${TARGET_TOOL}" "''${@}"
'';
checkPhase = ''
${pkgs.stdenv.shellDryRun} "$target"
${concatStringsSep "\n" (map link (attrNames tools))}
'';
}
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