Page 192 - DCAP310_INTRODUCTION_TO_ARTIFICIAL_INTELLIGENCE_AND_EXPERT_SYSTEMS

Introduction to Artificial Intelligence & Expert Systems

Notes 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.

Self Assessment

State whether the following statements are true or false:
1. Tree patterns are used in all programming languages as a general tool to process data
based on its structure.
2. Pattern matching sometimes include support for guards.
3. Sequence patterns are often described using regular expressions and matched using
techniques.

10.2 Measure for Matching

More complex patterns can be built from the primitive ones of the previous section, usually in
the same way as values are built by combining other values. The difference then is that with
variable and wildcard parts, a pattern doesn’t build into a single value, but matches a group of
values that are the combination of the concrete elements and the elements that are allowed to
vary within the structure of the pattern.
A tree pattern describes a part of a tree by starting with a node and specifying some branches and
nodes and leaving some unspecified with a variable or wildcard pattern. It may help to think of
the abstract syntax tree of a programming language and algebraic data types.
In Haskell, the following line defines an algebraic data type Color that has a single data
constructor ColorConstructor that wraps an integer and a string.
data Color = ColorConstructor Integer String
The constructor is a node in a tree and the integer and string are leaves in branches.

When we want to write functions to make Color an abstract data type, we wish to write functions
to interface with the data type, and thus we want to extract some data from the data type, for
example, just the string or just the integer part of Color.

If we pass a variable that is of type Color, how can we get the data out of this variable? For
example, for a function to get the integer part of Color, we can use a simple tree pattern and
write:
integerPart (ColorConstructor theInteger _) = theInteger
As well:
stringPart (ColorConstructor _ theString) = theString
The creations of these functions can be automated by Haskell’s data record syntax.

Filtering Data with Patterns

Pattern matching can be used to filter data of a certain structure. For instance, in Haskell a list
comprehension could be used for this kind of filtering:
[A x|A x <- [A 1, B 1, A 2, B 2]]
evaluates to
[A 1, A 2]

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