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| Topic: | Data definition of pic | |
| Remarks: | Helpful, and some important general concepts | |
| Relates to: | recursion, and composite data | |
| on-test? | not until we have data definitions officially | |
A data definition for pic:
a gentle intro to recursive data definitions...
When we start working with composite data, we will use data definitions. Because pic is a fairly fancy kind of data, this is not a very precise data definition, but it will get us started. There is a somewhat more formal definition of a pic in the big comment in the Teachpack's source code, piccy-graph.ss, if you are interested.
A pic is defined as either:
- empty-pic,
- the result of a function application that looks something like (empty-pic-w-title 300 300 "My Picture")
- the result of a graphic application that looks like: (GRAPHIC-OP OPERANDS PIC)
where
- GRAHICS-OP stands for either pt, dot, circle or some other calls
- OPERANDS stands for the operands other than the pic we are drawing on. For example circle will need num num num.
- PIC is the pic we are drawing on.
Note that there is a PIC in the definition of pic!
That seems weird, doesn't it. See the examples below, and then look at the purple definition again.
This is called a recursive definition. Recursion lets little definitions define big things. The important thing is that a recursive definition is precise enough to make computers (and certain kinds of people) happy. See how it works? Recursion is a pretty wonderful topic and we'll be learning lots more about it, believe you me.