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Invocations and Rules

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A predicate is used to specify a computation. Such a computation is triggered by a predicate invocation. For example, if we have defined a predicate favorite that determines for a given Person the favorite Color, then we can write


   favorite(P -> C)

to ask for the favorite color C of P.

A predicate is described by rules. For example, the predicate favorite could have been defined by the following rules:


   'rule' favorite (jim -> red)
   'rule' favorite (julia -> blue)
   'rule' favorite (jane -> red)
   'rule' favorite (john -> yellow)

An item like favorite (jim -> red) here is called a rule heading (rule bodies, here empty, are discussed later).

An invocation


   favorite (julia -> C)

would define the variable C as blue because the second rule defines the output blue for the input julia.

A predicate may have input and output parameters. The input parameters come first, the output parameters being separated by an arrow. Here is a predicate with no output parameters:


   IsJimsColor(Val)

with rule


   'rule' IsJimsColor(red)

and one with no input parameters:


   JimsColor(-> Val)

with rule


   'rule' JimsColor(-> red)

If a list of input or output parameters is supplied, a comma is used to separate the members.

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