Monday, February 18, 2008

Polymorphic repr

I'm not about to debate whether debugging is a valuable exercise in JavaScript, nor whether introspection or reflection tools are useful, nor whether they would be especially helpful in a dynamic language. Ruby has inspect. Python has repr. The Chiron JavaScript library has repr too.

Notionally, repr is the inverse of eval for a reasonable subset of JavaScript. There are a lot of object hierarchies that cannot be reconstructed from a repr serialization, but as a debugging tool, repr is indispensable.

repr is a polymorphic function you can import from base.js. If you pass repr an object that implements a repr member function, repr will defer to your overridable repr. Otherwise, repr returns reasonable defaults for other types. For example, repr provides defaults for Array, Object, String, Number, Boolean, and Date. repr also recursively represents members of arrays and objects, but provides circular reference protection by tracking visited objects in a memo Set.

Chiron's debugger uses repr to convert the value of an expression on the command line to a human-readable string.

j$ 1
j$ repr(1)
j$ "hi"
j$ repr("hi")
j$ true
j$ {a: 10}
{"a": 10}
j$ [1, 2, 3]
[1, 2, 3]
j$ [{a: 10}]
[{"a": 10}]
j$ var a = {}; a.a = a; a
{"a": <cycle>}
j$ type()()
<instance run.html#0 0>
j$ type({'repr': function () {return "x"}})()

No comments: