Evaluate user-defined functions

lib/chervil/ast/function.rb

     def ==(other)
       @params == other.params && @body == other.body
     end
+
+    def call(args)
+      unless @params.size == args.size
+        raise "Expected #{params.size} arguments but received #{args.size}"
+      end
+
+      env = ::Chervil::Env.new
+      @params.zip(args).each do |key, value|
+        env.set(key.name, value)
+      end
+
+      current_expr = nil
+      @body.each do |expr|
+        current_expr = expr.evaluate(env)
+      end
+
+      current_expr
+    end
   end
 end

spec/ast/function_spec.rb

+module Chervil
+  RSpec.describe AST::Definition do
+    it 'evaluates its body with its arguments when called' do
+      env = Env.new
+      expect(
+        AST::Function.new(
+          [
+            AST::Identifier.new("x")
+          ],
+          [
+            AST::Application.new(
+              AST::Identifier.new("*"),
+              [
+                AST::Identifier.new("x"),
+                AST::Identifier.new("x")
+              ]
+            )
+          ],
+        ).call(
+          [5.0]
+        )
+      ).to eq(25.0)
+    end
+  end
+end