Add ability to register template engine outside the library

README.md

@@ -6,12 +6,14 @@ Generic templating interface for Crystal.
 
 Simplify developers' lives by abstracting template rendering for multiple template languages.
 
-## Supported
+## Supported out of the box
 
 | Language | File extensions | Required libraries |
 | -------- | --------------- | ------------------ |
 | ECR      | .ecr            | none (part of the stdlib) |
-| Slang    | .slang          | [slang](https://github.com/jeromegn/slang) |
+
+See also:
+[Registering your own template engine](#registering-your-own-template-engine).
 
 ## Installation
 
@@ -55,6 +57,26 @@ end
 puts str # => <compiled template>
 ```
 
+## Registering your own template engine
+
+Use `Kilt.register_template(extension, embed_command)` macro:
+
+```crystal
+require "kilt"
+
+module MyEngine
+  macro embed(filename, io_name)
+    # ....
+  end
+end
+
+Kilt.register_template(".myeng", ::MyEngine.embed)
+```
+
+This can be part of your own `my-engine` library: in this case it should depend
+on `kilt` directly, or this could be a part of adapter library, like:
+`kilt-my-engine`, which will depend on both `kilt` and `my-engine`.
+
 ## Contributing
 
 Please contribute your own "adapter" if you create a template language for Crystal that's not yet supported here!

spec/spec_helper.cr

@@ -1,9 +1,11 @@
 require "spec"
 require "../src/kilt"
+
 require "slang"
+Kilt.register_template(".slang", embed_slang)
 
 macro render_file(filename)
   String.build do |__io__|
     Kilt.embed({{filename}}, "__io__")
   end
-end
+end

src/kilt.cr

@@ -2,17 +2,55 @@ require "ecr/macros"
 require "./kilt/*"
 
 module Kilt
+  # macro only constants
+  TEMPLATES = [] of Int32
+  OVERRIDES = [] of Int32
+
+  macro register_template(extension, embed_macro)
+    {% ::Kilt::TEMPLATES << {extension, embed_macro} %}
+    ::Kilt._override_embed
+  end
 
   macro embed(filename, io_name = "__kilt_io__")
-    {% if filename.ends_with?(".ecr") %}
-      embed_ecr({{filename}}, {{io_name}})
-    {% elsif filename.ends_with?(".slang") %}
-      embed_slang({{filename}}, {{io_name}})
-    {% else %}
-      raise Kilt::Exception.new("Unsupported template type \"" + {{filename.split(".").last}} + "\"")
-    {% end %}
+    {{::Kilt::OVERRIDES.last.id}}.embed({{filename}}, {{io_name}})
   end
 
+  macro _override_embed
+    {% override_name = "::Kilt::Override#{OVERRIDES.size}" %}
+
+    module {{override_name.id}}
+      macro embed(filename, io_name)
+        {% start = true %}
+
+        {% for template in ::Kilt::TEMPLATES %}
+          {% extension = template[0] %}
+          {% embed_macro = template[1] %}
+
+          {% if start == true %}
+            {% start = false %}
+
+            \{% if filename.ends_with?({{extension}}) %}
+              {{embed_macro.id}}(\{{filename}}, \{{io_name}})
+
+          {% else %}
+
+            \{% elsif filename.ends_with?({{extension}}) %}
+              {{embed_macro.id}}(\{{filename}}, \{{io_name}})
+
+          {% end %}
+        {% end %}
+
+            \{% else %}
+              raise Kilt::Exception.new("Unsupported template type \"" + \{{filename.split(".").last}} + "\"")
+            \{% {{:end.id}} %}
+      end
+    end
+
+    {% ::Kilt::OVERRIDES << override_name %}
+  end
+
+  register_template(".ecr", embed_ecr)
+
   macro file(filename, io_name = "__kilt_io__")
     def to_s({{io_name.id}})
       Kilt.embed({{filename}}, {{io_name}})