Guards
Guards are constructs that we can use to increase the power of
pattern matching. Using guards, we can perform simple tests and
comparisons on the variables in a pattern. Suppose we want to write
a function max(X, Y) that computes the max of X and
Y. We can write this using a guard as follows:
| | max(X, Y) when X > Y -> X; |
| | max(X, Y) -> Y. |
The first clause matches when X is greater than
Y and the result is X.
If the first clause doesn’t match, then the second clause is
tried. The second clause always returns the second argument
Y. Y must be greater than or equal to X;
otherwise, the first clause would have matched.
You can use guards in the heads of function definitions where they are introduced by the when keyword, or you can use them ...
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