[W/K] :: pattern matching

  Pattern matching \Pat"tern match"ing\, n. [See pattern.]
     (Computers)
     A technique in automated data analysis, usually performed on
     a computer, by which a group of characteristic properties of
     an unknown object is compared with the comparable groups of
     characteristics of a set of known objects, to discover the
     idenity or proper classification of the unknown object.
  
     Note: There are two major types of pattern matching,
           statistical pattern matching and syntactic pattern
           matching. In statistical pattern matching, the
           criteria used to recognize identity or class membership
           vary, but in general some combination of the
           differences in the groups of characteristics of known
           and unknown objects are considered to be a measure of
           the difference ("distance") between them, and the
           closest known object or objects are viewed as
           presenting the most likely identity or class for the
           unknown object. In syntactic pattern matching, a set of
           known patterns, e.g. as in the possible order of parts
           of speech in a language, is defined, and the unknown
           pattern is compared to find that known pattern or
           patterns which matches the unknown exactly. In general,
           statistical pattern matching is used where properties
           of objects with continuous values are being compared,
           and syntactic pattern matching where a complex
           arrangement of at least two different objects may be
           built by application of a set of rules (a "grammar")
           for combining the objects in a specified order.
           Examples of the latter are natural and formal
           languages.
           [PJC]

  pattern matching
       
          1. A function is defined to take arguments of a particular
          type, form or value.  When applying the function to its actual
          arguments it is necessary to match the type, form or value of
          the actual arguments against the formal arguments in some
          definition.  For example, the function
       
          	length []     = 0
          	length (x:xs) = 1 + length xs
       
          uses pattern matching in its argument to distinguish a null
          list from a non-null one.
       
          There are well known algorithm for translating pattern
          matching into conditional expressions such as "if" or "case".
          E.g. the above function could be transformed to
       
          	length l = case l of
          		   []   -> 0
          		   x:xs -> 1 : length xs
       
          Pattern matching is usually performed in textual order though
          there are languages which match more specific patterns before
          less specific ones.
       
          2. Descriptive of a type of language or utility such as awk
          or Perl which is suited to searching for strings or patterns
          in input data, usually using some kind of regular
          expression.
       
          (1994-11-28)