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Match-Bind Expression

The match-bind expression performs a pattern match, and tests whether it succeeds. It evaluates the left hand expression to a value -- the specimen. It then matches the specimen against the pattern. The match-bind expression as a whole evaluates to a boolean indicating whether the match succeeded. In either case, the variable bound by pattern are available in the succeeding scope.

eExpr "=~" pattern
<!ELEMENT matchBindExpr (%eExpr;, %pattern;)>
The pattern is expanded to produce a boolean saying whether we have a match. Only when we succeed do we bind the variable in the pattern to extracts of the specimen. If we fail, all these variable must be broken in the following scope. Therefore, we use temporary standins for these variable during the match.
list =~ [car] + cdr
in Kernel-E:
list =~ [car :any] + cdr :any
in XML:
in Java:
boolean result_4 = false;

Object car = Ref.broken(...);
Object cdr = Ref.broken(...);
fail_4: {
    if (! isTuple(list)) { break fail_4; }

    Integer len = E.toInteger(, "size"));
    if (len.intValue() < 1) { break fail_4; }
    final Object car_tmp =, "get", 0);
    final Object cdr_tmp =, "run", 1, len);

    result_4 = true;
    car = car_tmp;
    cdr = cdr_tmp;
//result_4 is the value of the expression

If the match succeeded, then these variables will bound to the values they extracted from the specimen. If the match fails, then, in the succeeding scope, all these variables will be bound to broken references (XXX need to specify the problem). If a variable was part of an earlier sub-pattern than succeeded before the match as a whole failed, then this variable will be seen to bound to the value it extracted from the specimen by later code within the overall pattern, but will be seen to by bound to the broken reference by code after the match-bind expression as a whole. This "change" in binding happens even if the variable is declared ":final". Effectively, the variables defined within the pattern are redefined following the pattern and initialized accordingly.
Unless stated otherwise, all text on this page which is either unattributed or by Mark S. Miller is hereby placed in the public domain.
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