Pattern Matching - Primitive Patterns

Primitive Patterns

The simplest pattern in pattern matching is an explicit value or a variable. For an example, consider a simple function definition in Haskell syntax (function parameters are not in parentheses but are separated by spaces, = is not assignment but definition):

f 0 = 1

Here, 0 is a single value pattern. Now, whenever f is given 0 as argument the pattern matches and the function returns 1. With any other argument, the matching and thus the function fail. As the syntax supports alternative patterns in function definitions, we can continue the definition extending it to take more generic arguments:

f n = n * f (n-1)

Here, the first n is a single variable pattern, which will match absolutely any argument and bind it to name n to be used in the rest of the definition. In Haskell (unlike at least Hope), patterns are tried in order so the first definition still applies in the very specific case of the input being 0, while for any other argument the function returns n * f (n-1) with n being the argument.

The wildcard pattern (often written as _) is also simple: like a variable name, it matches any value, but does not bind the value to any name.

Read more about this topic:  Pattern Matching

Famous quotes containing the words primitive and/or patterns:

    That primitive head
    So ambitiously vast,
    Yet so rude in its art,
    Is as easily read
    For the woes of the past
    As a clinical chart.
    Robert Frost (1874–1963)

    Teasing is universal. Anthropologists have found the same fundamental patterns of teasing among New Zealand aborigine children and inner-city kids on the playgrounds of Philadelphia.
    Lawrence Kutner (20th century)