ITEC 380 2009fall ibarland

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lect03b
union types

Review:
We saw re-writing OO as static methods, adding implicit param `this`.

We saw types as sets;
Cf.   n : int
      f : String, double -> int
with
       n ∈ Z
       f ∈ Σ* ✗ R → Z

objects/structs are cross-product of types



Today: union types
e.g., LibItem = Book U CD

CDs: have titles, and a time (in minutes)

Let's write a functions for any LibItem.

How to write: 
 pass-time : LibItem -> number

(What is the template?)

Updating The Design Recipe
(Adapted from How to Design Programs)
For each type of data involved:
    1. Data Definition (how will you represent an age, a gender, a book, etc.)
    2. Examples of the Data   (trivial first)
Then, for each function involving that data:
    3. description, signature, and header/stub
    4. test cases
    5a.  case analysis
    5b. selectors  (what is the type of each?)
    5c. add any natural/pertinent function calls on these pieces
    5d. assemble the parts of 5b.  
    6. Run tests.




Coming up:
- grammars (context-free):
    sentence3
    parsing; showing a derivation.

    grammar w/ a loop: 'sentence*'

    grammar for scheme we've seen

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©2009, Ian Barland, Radford University Last modified 2009.Sep.16 (Wed)

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