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kojix2

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Embedding the Crystal Compiler in Your Program

#crystal

Introduction

The Crystal compiler can be used as a library.
This document explains how to set it up and use it.

Creating the Project

First, create a new Crystal project.

crystal init app duck_egg
cd duck_egg
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Editing shard.yml

Edit the shard.yml file as follows.
In this example, we add markd and reply to the dependencies section.

name: duck_egg
version: 0.1.0

targets:
  πŸ₯š:
    main: src/duck_egg.cr

dependencies:
  markd:
    github: icyleaf/markd
  reply:
    github: I3oris/reply
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Creating duck_egg.cr

Create src/duck_egg.cr and add the following code.

require "compiler/requires"

BIRDS = [
  { "πŸ”", "cluck!" },
  { "πŸ“", "cock-a-doodle-doo" },
  { "πŸ¦ƒ", "gobble" },
  { "πŸ¦†", "quack" },
  { "πŸ¦‰", "hoot" },
  { "🦜", "squawk" },
  { "πŸ•Š", "coo" },
  { "🦒", "honk" },
  { "🦩", "brrrrt" },
  { "🐧", "honk honk" },
  { "🦀", "boop" },
  { "πŸ¦•", "Bwooooon!!" },
  { "πŸ¦–", "Raaaaawr!!" }
]

bird, sound = BIRDS.sample

compiler = Crystal::Compiler.new
source = Crystal::Compiler::Source.new(bird, %Q(puts "#{bird}  < #{sound}"))
compiler.compile source, bird
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In this program, the Crystal compiler is embedded in the target πŸ₯š.
When πŸ₯š is executed, a random bird is selected.
The embedded compiler generates a binary that displays the bird and its sound.

Building and Running

First, build the program.

shards build
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Next, check the CRYSTAL_PATH environment variable to find the location of the Crystal standard library.

crystal env
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The Crystal compiler requires the standard library even for very simple code such as puts 0.
Therefore, CRYSTAL_PATH must be set to include the path to the standard library.

export CRYSTAL_PATH=lib:/usr/local/bin/../share/crystal/src
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Run the program:

bin/πŸ₯š
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Example output:

πŸ¦–
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Run the generated binary:

./πŸ¦–
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Output:

πŸ¦–  < Raaaaawr!!
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Summary

By using the Crystal compiler as a library, you can generate and compile code dynamically. This technique can be applied in many interesting ways.

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