useslib is a litle macro for checking if a user has a C dependency installed. It's a way for c-binding authors to alert their users that they need to have some dependencies installed without giving them difficult to understand compiler errors.

If the user has the item, then compilation will work like normal. If they don't, the project will throw a normal crystal compiler error with a message like this:

❯ crystal run src/useslib.cr
Error in src/useslib.cr:14: Unable to find c library 'fuzzboing' on your system. Are you sure you installed it?

useslib fuzzboing
^~~~~~~

Without useslib, a @[Lib("imagemagick")] call will throw an error like this:

ld: library not found for -lMagickWand
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Error: execution of command failed with code: 1: `cc "${@}" -o '/Users/flaque/.cache/c
rystal/crystal-run-webimg.tmp'  -rdynamic  -lMagickWand -lpcre -lgc -lpthread /usr/loc
al/Cellar/crystal-lang/0.24.2_1/src/ext/libcrystal.a -levent -liconv -ldl -L/usr/lib -
L/usr/local/lib`

That's a lot harder to figure out.

pkg-config probably won't work on every system and I've only tested it so far on macOS. It's probably not a good idea to use this for everything. If you find issues with the project on other OS's please let me know.

Add this to your application's shard.yml:

dependencies:
  useslib:
    github: flaque/useslib

Just put useslib someLibrary at the top of your file.

require "useslib"

useslib imagemagick

puts "this code will only get to the compiler if imagemagick is installed"

1. Fork it ( https://github.com/flaque/useslib/fork )
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request

• flaque Flaque - creator, maintainer