Added some addition type-safety checks.

lib/template.js

@@ -38,10 +38,18 @@ module.exports = function(template) {
           return parse(expr[n.slice(1)|0]);
         });
 
-      else if(typeof template[expr[0]] === "function")
-        return template[expr[0]].apply(null, expr.slice(1).map(function(arg) {
-          return parse(arg);
-        }));
+      else if(typeof template[expr[0]] === "function") {
+        if(template[expr[0]].length === 0)
+          throw new Error("\"" + expr[0] + "\" was used in a template context, but is expected in a value context.");
+
+        else if(template[expr[0]].length !== expr.length - 1)
+          throw new Error("Template \"" + expr[0] + "\" did not expect " + (expr.length - 1) + " arguments.");
+
+        else
+          return template[expr[0]].apply(null, expr.slice(1).map(function(arg) {
+            return parse(arg);
+          }));
+      }
 
       else
         throw new Error("\"" + expr[0] + "\" is not a valid language template pattern.");

test.js

@@ -42,6 +42,10 @@ describe("translation", function() {
       expect(convert(["bar", 10, 20])).to.equal("meeple 20");
     });
 
+    it("should fail to apply a function template given the wrong number of arguments", function() {
+      expect(function() { convert(["baz", 10, 20, 30]); }).to.throw();
+    });
+
     it("should apply a function template", function() {
       expect(convert(["baz", 10, 20])).to.equal("meeple 20");
     });
@@ -68,6 +72,10 @@ describe("translation", function() {
       expect(convert("quux")).to.equal("glorple");
     });
 
+    it("should fail to apply a zero-argument function given arguments", function() {
+      expect(function() { convert(["quux"]); }).to.throw();
+    });
+
     it("should fail to apply a function template given a value", function() {
       expect(function() { convert("baz"); }).to.throw();
     });