Introducing Erlang

Chapter 4. Logic and Recursion

So far, Erlang seems logical but fairly simple. Pattern matching controls the flow through a program, and requests that match a form return certain responses. While this is enough to get many things done, there are times when you’ll want more powerful options, especially as you start working with larger and more complicated data structures.

Logic Inside of Functions

Pattern matching and guards are powerful tools, but there are times when it’s much easier to do some comparisons inside of a function clause instead of creating new functions. Erlang’s designers agreed, and created two constructs for evaluating conditions inside of functions: the case expression and the less frequently used if expression.

The case construct lets you use pattern matching and guards inside of a function clause. It reads most clearly when a single value (or set of values) needs to be compared with multiple possibilities. The if construct evaluates only a series of guards, without pattern matching. The if construct tends to produce more readable code in situations where the multiple possibilities are specified by combinations of different values.

Both constructs return a value your code can capture.

Evaluating Cases

The case construct lets you perform pattern-matching inside of your function clause. If you found the multiple function clauses of Example 3-2 hard to read, you might prefer to create a version that looks like Example 4-1, which you can find in ch04/ex1-case.

Example 4-1. Moving ...