Checkin the first stab at this.

Author: wyhaines

.editorconfig

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+root = true
+
+[*.cr]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+indent_style = space
+indent_size = 2
+trim_trailing_whitespace = true

.github/workflows/ci.yml

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+name: define.cr CI
+
+on:
+  schedule:
+    - cron:  '0 3 * * *'
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    container:
+      image: crystallang/crystal
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install dependencies
+      run: shards install
+    - name: Run tests
+      run: crystal spec -t -s
+    - name: Run Ameba
+      run: bin/ameba
+    - name: Build docs
+      run: crystal docs
+    - name: Deploy
+      uses: peaceiris/actions-gh-pages@v3
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        publish_dir: ./docs

.gitignore

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+/docs/
+/lib/
+/bin/
+/.shards/
+*.dwarf
+
+# Libraries don't need dependency lock
+# Dependencies will be locked in applications that use them
+/shard.lock

README.md

@@ -1,2 +1,54 @@
-# defined.cr
-This is a very small shard that  just provides a macro that can be used to see if a given type/constant is defined. The main use case would be for a library that might choose to do different things depending on whether another, optional library has been required or not.
+# defined
+
+![Defined.cr CI](https://img.shields.io/github/workflow/status/wyhaines/defined.cr/defined.cr%20CI?style=for-the-badge&logo=GitHub)
+[![GitHub release](https://img.shields.io/github/release/wyhaines/defined.cr.svg?style=for-the-badge)](https://github.com/wyhaines/defined.cr/releases)
+![GitHub commits since latest release (by SemVer)](https://img.shields.io/github/commits-since/wyhaines/defined.cr/latest?style=for-the-badge)
+
+This is a very small shard that provides a single macro, *defined?()*, which returns *VALUE*/*false* depending on whether the argument provided points to an existing constant. All lookups start at the top level and descend from there.
+
+## Installation
+
+1. Add the dependency to your `shard.yml`:
+
+   ```yaml
+   dependencies:
+     defined:
+       github: wyhaines/defined
+   ```
+
+2. Run `shards install`
+
+## Usage
+
+```crystal
+require "defined"
+```
+
+```crystal
+if defined?("OptionalFeature")
+  # The OptionalFeature library has been included into the code, so use it.
+else
+  # It's not there.
+end
+
+stuff = defined?("My::Stuff::Library") || Array(String)
+```
+
+## Development
+
+TODO: Write development instructions here
+
+## Contributing
+
+1. Fork it (<https://github.com/wyhaines/defined/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
+
+## Contributors
+
+- [Kirk Haines](https://github.com/wyhaines) - creator and maintainer
+
+![GitHub code size in bytes](https://img.shields.io/github/languages/code-size/wyhaines/defined.cr?style=for-the-badge)
+![GitHub issues](https://img.shields.io/github/issues/wyhaines/defined.cr?style=for-the-badge)

shard.yml

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+name: defined
+version: 0.1.0
+
+authors:
+  - Kirk Haines <[email protected]>
+
+crystal: "*"
+
+license: MIT
+
+development_dependencies:
+  ameba:
+    github: crystal-ameba/ameba
+    version: ~> 0.14

spec/defined_spec.cr

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+require "./spec_helper"
+
+describe Defined do
+  it "can find an existing constant, specified with a Symbol" do
+    defined?(:String).should be_truthy
+  end
+
+  it "can find an existing constant, specified with a String" do
+    defined?("Int::Primitive").should be_truthy
+  end
+
+  it "returns false when something isn't defined" do
+    defined?("This::Thing::Does::Not::Exist").should be_false
+  end
+
+  it "deals with a :: at the start of the constant name" do
+    defined?("::Foo::Bar").should be_false
+    defined?("::Hash").should be_truthy
+  end
+end

spec/spec_helper.cr

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+require "spec"
+require "../src/defined"

src/defined.cr

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+module Defined
+  VERSION = "0.1.0"
+end
+
+# This macro accepts a string or a symbol of a fully qualified constant name.
+# It validates whether the constant is defined, starting at the top level. The value
+# of the constant will be returned if it is defined. If it is not defined, `false` is
+# returned.
+macro defined?(const)
+  {%
+    parts = [] of String
+    position = @type
+    const.id.gsub(/^::/,"").split("::").all? do |part|
+      clean_part = part.tr(":","").id
+      parts << clean_part
+      if position && position.has_constant?(clean_part.id)
+        position = position.constant(clean_part.id)
+      else
+        position = false
+      end
+    end
+  %}
+  {% if position %}
+    {{ parts.join("::").id }}
+  {% else %}
+    false
+  {% end %}
+end