ARQ - Assignment

ARQ includes support for an explicit assignment of variables.

This involves is syntactic extension and is available is the query is parsed with language Syntax.syntaxARQ.

See also SELECT expressions which is also a form of assignment.

Assignment

The general form is:

LET ( variable := expression )

For example:

LET ( ?x := 2 )

{ ?x :name ?name .
  LET ( ?age2 := ?age - 21 )

Note: Assignment is ":="

Assignment Rules

ARQ assignment is single assignment, that is, once a variable is assigned a binding, then it can not be changed in the same query solution.

Only one LET expression per variable is allowed in a single scope.

The execution rules are:

• If the expression does not evaluate (e.g. unbound variable in the expression), no assignment occurs and the query continues.
• If the variable is unbound, and the expression evaluates, the variable is bound to the value.
• If the variable is bound to the same value as the expression evaluates, nothing happens and the query continues.
• If the variable is bound to a different value as the expression evaluates, an error occurs and the current solution will be excluded from the results.

Note that "same value" means the same as applies to graph pattern matching, not to FILTER expressions. Some graph implementation only provide same-term graph pattern matching. FILTERs always do value-based comparisons for "=" for all graphs.

Use with CONSTRUCT

One use is to perform some calculation prior to forming the result graph in a CONSTRUCT query.

CONSTRUCT { ?x :lengthInCM ?cm }
WHERE
{
   ?x :lengthInInches ?inch .
   LET ( ?cm := ?inches/2.54 )
}

Use with !BOUND

The OPTIONAL/!BOUND/FILTER idiom for performing limited negation of a pattern in SPARQL can be inconvenient because it requires a variable in the OPTIONAL to be assigned by pattern matching.  Using a LET can make that easier; here, we assign to ?z (any value will do) to mark when the matching pattern included the OPTIONAL pattern.

Example: ?x with no ":p 1" triple:

{
  ?x a :FOO .
  OPTIONAL { ?x :p 1 . LET (?z := true) }
  FILTER ( !BOUND(?z) )
}

Note that negation is supported properly through the NOT EXISTS form.

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