Add cons, head, and tail; remove type checking

I think there’s a better way to do type checking so I’m going to hold off on it for now and get basic functionality working for now and come back later.

src/core.js

@@ -12,14 +12,8 @@ module.exports = {
   '>=': (a, b) => new AST.Boolean({ value: a.value >= b.value }),
   '<=': (a, b) => new AST.Boolean({ value: a.value <= b.value }),
   'list': (...args) => { return new AST.List({ elements: args }) },
-  'length': xs => {
-    if (xs.constructor == AST.List) {
-      return new AST.Number({ value: xs.elements.length })
-    }
-
-    return new OsloError({
-      line: xs.line,
-      message: 'length: argument is not a list'
-    })
-  }
+  'length': xs => new AST.Number({ value: xs.elements.length }),
+  'cons': (x, xs) => new AST.List({ elements: [x, ...xs.elements] }),
+  'head': xs => xs.elements[0],
+  'tail': xs => new AST.List({ elements: xs.elements.slice(1) }),
 }

test/coreTest.js

@@ -18,17 +18,28 @@ test('list function', t => {
 })
 
 test('length function', t => {
-  t.plan(2)
+  t.plan(1)
 
-  let tree = helpers.evaluate('(length (list 1 2 3))')
+  const tree = helpers.evaluate('(length (list 1 2 3))')
   t.deepEqual(tree[0], new AST.Number({ value: 3 }))
+})
+
+test('map', t => {
+  t.plan(1)
+  const tree = helpers.evaluate(`
+    (define my-map
+      (lambda (f xs)
+        (if (= (length xs) 0)
+          (list)
+          (cons
+            (f (head xs))
+            (my-map f (tail xs))))))
 
-  tree = helpers.evaluate('(length 5)')
-  t.deepEqual(
-    tree,
-    new OsloError({
-      line: 1,
-      message: 'length: argument is not a list'
-    })
-  )
+    (my-map (lambda (x) (+ x 1)) (list 1 2 3))
+  `)
+  t.deepEqual(tree[0], new AST.List({ elements: [
+    new AST.Number({ value: 2 }),
+    new AST.Number({ value: 3 }),
+    new AST.Number({ value: 4 }),
+  ]}))
 })