Members

The body of a rule is given by a sequence of members. A member is elaborated as follows.

Predicate Invocations

A member of the form

• p ( E₁ , ... , E_n -> P₁ , ... , P_m )

p must be a predicate with n input parameters and m output parameters. Let T₁, ..., T_n be the input types and S₁, ..., S_m the output types. The expressions E₁, ..., E_n appear on positions with types T₁, ..., T_n, and the patterns P₁, ..., P_m appear on positions with types S₁, ..., S_m.

The expressions E₁, ..., E_n are evaluated, yielding values Tau₁, ..., Tau_n. p is invoked with these input values. If the invocation fails, the member M fails. If it succeeds, it yields output values Sigma₁, ..., Sigma_m. These values are matched against the patterns P₁, ..., P_m. If the matching fails, the member M fails. Otherwise, the member succeeds and the variables appearing in P₁, ..., P_m are defined.

Context Updates

A member of the form

• D <- E

The value of D is defined as the value of the expression E. If T is the type of D, E appears on a position with type T.

The context designator D may be of the form C, where C must be declared as a context variable. The type of the designator is the type of the context variable.

	Example NestingLevel <- N This sets the value of the context variable NestingLevel to the value of the local variable N.

A context designator may also have the form I ' F ; I must be a variable of the key type of a context table T. Entries of T must have a field F. The type of the designator is the type of the F.

	Example Let Lab be a local variable that refers to a table entry with a field Coordinate. Then Lab'Coordinate <- Loc sets this field to the value of Loc.

Context Queries

• D -> P

The value of the context designator D is matched against the pattern P. If this succeeds, the member succeeds and the variables appearing in P are defined. Otherwise, the member fails.

	Examples NestingLevel -> N This defines the local variable N according to the current value of the context variable NestingLevel. Let Lab be a local variable that refers to a table entry with a field Coordinate. Then Lab'Coordinate -> Loc defines the local variable Loc according to the value of this field.

It is a checked run-time error to access an undefined context designator.

Key Definitions

• V :: T

The variable V is defined as a unique key of type T.

	Example Lab :: Label The value of the local variable Lab is the unique key of a newly created entry of table Label.

Anonymous Tokens

A member of the form

• S

Alternative Statements

A member of the form

• (| A₁ || ... || A_n |)

The alternatives A₁, ..., A_n are evaluated from left to right until one alternative A_i succeeds. Then M succeeds. If all alternatives fail, M fails.

An alternative A_i has the form M_i,1, ..., M_i,ni. These members are evaluated from left to right until one member fails. Then the alternative fails. If all members succeed, the alternative succeeds.

A variable that is defined in all alternatives A₁, ..., A_n is defined by the alternative statement and may be used outside it. Such a variable must be defined with the same type in all alternatives.

A variable that is defined in one but not all alternatives is local to that alternative.

Conditional Statements

• [| A |]

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