Add YAML support

Author: Sija

README.md

@@ -117,6 +117,22 @@ example.to_json
 # => "{\"id\":\"aWgEPTl1tmebfsQzFP4bxwgy80V\"}"
 ```
 
+### YAML
+
+```crystal
+require "ksuid/yaml"
+
+class Example
+  YAML.mapping id: KSUID
+end
+
+example = Example.from_yaml(%(---\nid: aWgEPTl1tmebfsQzFP4bxwgy80V\n))
+# => #<Example:0x10a8723c0 @id=KSUID(aWgEPTl1tmebfsQzFP4bxwgy80V)>
+
+example.to_yaml
+# => "---\nid: aWgEPTl1tmebfsQzFP4bxwgy80V\n"
+```
+
 ## Contributing
 
 1. Fork it (<https://github.com/Sija/ksuid.cr/fork>)

spec/ksuid/yaml_spec.cr

@@ -0,0 +1,16 @@
+require "../spec_helper"
+require "../../src/ksuid/yaml"
+
+private class YAMLWithKSUID
+  YAML.mapping value: KSUID
+end
+
+describe KSUID do
+  context "YAML mapping" do
+    it "parses KSUID from YAML" do
+      ksuid = YAMLWithKSUID.from_yaml(%(---\nvalue: 0o5Fs0EELR0fUjHjbCnEtdUwQe3\n))
+      ksuid.should be_a(YAMLWithKSUID)
+      ksuid.value.should eq(KSUID.from("0o5Fs0EELR0fUjHjbCnEtdUwQe3"))
+    end
+  end
+end

src/ksuid/yaml.cr

@@ -0,0 +1,33 @@
+require "yaml"
+require "../ksuid"
+
+# Adds YAML support to `KSUID` for use in a YAML mapping.
+#
+# NOTE: `require "ksuid/yaml"` is required to opt-in to this feature.
+#
+# ```
+# require "ksuid"
+# require "ksuid/yaml"
+#
+# class Example
+#   YAML.mapping id: KSUID
+# end
+#
+# example = Example.from_yaml(%(---\nid: aWgEPTl1tmebfsQzFP4bxwgy80V\n))
+#
+# ksuid = KSUID.from("0o5Fs0EELR0fUjHjbCnEtdUwQe3")
+# ksuid.to_yaml # => "--- 0o5Fs0EELR0fUjHjbCnEtdUwQe3\n"
+# ```
+
+struct KSUID
+  def self.new(ctx : YAML::ParseContext, node : YAML::Nodes::Node)
+    unless node.is_a?(YAML::Nodes::Scalar)
+      node.raise "Expected scalar, not #{node.class}"
+    end
+    from(node.value)
+  end
+
+  def to_yaml(yaml : YAML::Nodes::Builder)
+    yaml.scalar(to_s)
+  end
+end