Pattern calculus bases all computation on pattern matching of a very general kind. Like lambda calculus, it supports a
uniform treatment of function evaluation. Also, it allows functions to be
passed as arguments and returned as results. In addition, pattern calculus supports
uniform access to the internal structure of arguments, be they pairs
or lists or trees. Also, it allows patterns to be passed as arguments and
returned as results. Uniform access is illustrated by a
pattern-matching function that computes the size of an
arbitrary data structure. In the notation of the programming language
bondi, it is given by the recursive function
let rec size = 
 | x y - ; (size x) + (size y) 
 | x - ; 1 
The second, or "default case" matches the pattern 
against the argument and returns . This
case is used only if the matching failed in the first case. The
first, or "special case" matches against any "compound", such
as a non-empty list, or pair. Matching binds to the left component
and to the right component. Then the body of the case adds the
sizes of these components together.
Similar techniques yield generic queries for searching and updating. Combining recursion and decomposition in this way yields "path polymorphism".
The ability to pass patterns as parameters ("pattern polymorphism") is illustrated by defining a 
generic eliminator. Suppose given constructors for creating
the leaves of a tree, and for converting numbers into
counters. The corresponding eliminators are then
elimLeaf = | Leaf y - ; y 
elimCount = | Count y - ; y
For example, evaluates to as does .
These examples can be produced by applying the generic eliminator
 to the constructors in question. It is defined by
elim = | x - ; | {y} x y - ; y 
Now evaluates to codice_1 which is equivalent to . Also is equivalent to .
In general, the curly braces contain the bound variables of the
pattern, so that is free and is bound in codice_2.
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