structable params

lib/dentaku/type/declared.rb

@@ -18,10 +18,30 @@ def self.declare(name, arity=0, &b)
     end
 
     class DeclaredType
-      def self.arity; raise "abstract"; end
-      def arity; self.class.arity; end
+      class << self
+        def inspect
+          "DeclaredType(:#{type_name}/#{arity})"
+        end
+
+        def arity; raise "abstract"; end
+        def type_name; raise "abstract"; end
 
-      def self.type_name; raise "abstract"; end
+        def structable(keys={})
+          @structable_keys ||= {}
+
+          keys.each do |k, v|
+            @structable_keys[k.to_s] = v
+          end
+
+          @structable_keys
+        end
+
+        def structable?
+          !!@structable_keys
+        end
+      end
+
+      def arity; self.class.arity; end
       def type_name; self.class.type_name; end
 
       attr_reader :args

lib/dentaku/type/solver.rb

@@ -123,7 +123,13 @@ def simplify_expr(constraint, expr)
         expr.cases(
           key_of: ->(struct, key) {
             struct.cases(
-              param: -> { simplify_error!(expr) },
+              param: ->(name, args) {
+                as_struct = DECLARED_TYPES[name].structable
+
+                simplify_error!(expr, KeyError.new(constraint)) unless as_struct.key?(key)
+
+                Expression.from_sexpr(as_struct[key])
+              },
               struct: ->(keys, types) {
                 idx = keys.find_index(key)
                 return simplify_error!(expr, KeyError.new(constraint)) if idx.nil?

spec/type/declared_spec.rb

@@ -0,0 +1,25 @@
+require 'spec_helper'
+
+describe 'Declared Types' do
+  Dentaku::Type.declare(:fake_struct) do
+    structable(a: ':numeric', b: ':numeric')
+  end
+
+  Dentaku::Type.declare(:recursive) do
+    structable(a: ':recursive', b: ':numeric')
+  end
+
+  it 'destructs a structable type' do
+    checker = Dentaku::Type::StaticChecker.new(x: :fake_struct)
+    ast = Dentaku::Syntax.parse('x.a.a.a.a.b')
+
+    checker.check!(ast)
+  end
+
+  it 'recursively destructs a structable type' do
+    checker = Dentaku::Type::StaticChecker.new(x: :fake_struct)
+    ast = Dentaku::Syntax.parse('x.a.a.a.a.b')
+
+    checker.check!(ast)
+  end
+end