Angel \”Java\” Lopez on Blog

December 15, 2011

AjLisp in Ruby (2) Context with Name and Values

Previous Post

One of the first classes I implemented in AjLispRb is the environment. This time I called it a context: a dictionary where to save name/values, to store the atom values. The code:

module AjLisp
class Context
    def initialize(parent = nil)
        @parent = parent
        @values =
    def getValue(name)
        if @values.has_key?(name)
            return @values[name]
        if @parent != nil
            return @parent.getValue(name)
        return nil
    def setValue(name, value)
        @values[name] = value

The class was coded using TDD, the first tests at test/test_context.rb. Some tests:

def test_not_defined_is_nil
    context =
def test_set_and_get_value
    context =
    context.setValue(:foo, "bar")
    assert_equal("bar", context.getValue(:foo))
def test_get_value_from_parent
    parent =
    parent.setValue(:foo, "bar")
    context =
    assert_equal("bar", context.getValue(:foo))

Initially, I used strings for names, but now I’m using Ruby symbols like :foo. I have a test that ensures the independence of parent and child context values:

def test_override_value_from_parent
    parent =
    parent.setValue(:foo, "bar")
    context =
    context.setValue(:foo, "bar2")
    assert_equal("bar2", context.getValue(:foo))
    assert_equal("bar", parent.getValue(:foo))

Each context can have a parent. There is a top context, defined in lib/ajlisp.rb:

module AjLisp
@context =
@context.setValue :quote, FPrimitiveQuote.instance
@context.setValue :first, PrimitiveFirst.instance
@context.setValue :rest, PrimitiveRest.instance
@context.setValue :cons, PrimitiveCons.instance
@context.setValue :list, PrimitiveList.instance
@context.setValue :lambda, FPrimitiveLambda.instance
@context.setValue :flambda, FPrimitiveFLambda.instance
@context.setValue :mlambda, FPrimitiveMLambda.instance
@context.setValue :let, FPrimitiveLet.instance
@context.setValue :define, FPrimitiveDefine.instance
@context.setValue :do, FPrimitiveDo.instance
@context.setValue :if, FPrimitiveIf.instance
@context.setValue :definef, FPrimitiveDefinef.instance
@context.setValue :definem, FPrimitiveDefinem.instance
@context.setValue :+, PrimitiveAdd.instance
@context.setValue :-, PrimitiveSubtract.instance
@context.setValue :*, PrimitiveMultiply.instance
@context.setValue :/, PrimitiveDivide.instance
def self.context
    return @context
# ...

New context are created in many primitives. At the begin, there is a top context like:

If you have a defined form that returns the second element of a list:

(define second (a) (first (rest a)))

now you have a top context with the new slot:

The top context has a new slot named second with a lambda value (lambda (a) (first (rest a))

When you invoke:

(second (quote (one two three)))

during the invocation a new context is created, its parent set to the top context, and having a new name/value pair:

That context is discarded after second evaluation (unless the context were referenced by recently created closure, details in upcoming post). When you define a simple value:

(define one 1)

the top context is modified, to have a new name/value pair:

Next topics: primitives and special form primitives, their invocation, closures, macros, native methods invocation.

Keep tuned!

Angel “Java” Lopez

1 Comment »

  1. […] by AjLisp in Ruby (2) Context with Name and Values « Angel “Java” Lopez on Blog — December 15, 2011 @ 5:41 pm […]

    Pingback by AjLisp in Ruby (1) Structure, Classes and Tests « Angel “Java” Lopez on Blog — December 15, 2011 @ 5:43 pm

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