Thursday, March 8, 2012

Seven Languages in Seven Weeks - Io Day 1 Self-Study

Here's the first self-study section about the Io programming language. I didn't know Io before, I still find it a bit awkward, but I'm beginning to like it.

Evaluate 1 + 1 and then 1 + "one". Is Io strongly typed or weakly typed? Support you answer with code.
Io> 1 + 1
==> 2
Io> 1 + "one"

  Exception: argument 0 to method '+' must be a Number, not a 'Sequence'
  ---------
  message '+' in 'Command Line' on line 1
Io is strongly typed, operands of different types will not be implicitly converted:
Io> 1 + "2"

  Exception: argument 0 to method '+' must be a Number, not a 'Sequence'
  ---------
  message '+' in 'Command Line' on line 1
Is 0 true or false? What about the empty string? Is nil true or false? Support your answer with code.
Io> 0 and true
==> true
Io> "" and true
==> true
Io> nil and true
==> false
How can you tell what slots a prototype supports?

Invoking an object in the prompt will print the supported slots:
Io> Car
==>  Car_0x7fb039c9a1f0:
  type             = "Car"
  vroom            = method(...)
The slotNames message returns the list of slots:
Io> Car slotNames
==> list(type, vroom)
To get the slots defined by the parent, call the proto message:
Io> Car proto
==>  Vehicle_0x7fb039c6e270:
  description      = "Something that takes you far away"
  type             = "Vehicle"
What is the difference between = (equals), := (colon equals), and ::= (colon colon equals)? When would you use each one?
=Assigns value to slot if it exists, otherwise raises exception
:=Creates slot, assigns value
::=Creates slot, creates setter, assigns value
Run an Io program from a file
$ echo '"Hello, world!" println' > helloworld.io
$ io helloworld.io 
Hello, world!
Execute the code in a slot given its name

Unfortunately, I haven't been able to solve this one yet. I guess the intent is given a slot holding some code as a string, one should figure out a statement to execute this code.
Io> code := "\"hello\" println"
==> "hello" println