Ignore Pattern

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The ignore pattern always successfully matches against anything and does nothing.

BNF:

"_"

XML DTD:

<!ELEMENT ignorePattern EMPTY>

Java:

Pattern match implicitly succeeds without generating any code

Example:

Checks that we have a pair while extracting the first element and ignoring the second element.

def [first, _] := pair

in Kernel-E:

def [first :any, _] := pair

in XML:

<defineExpr>
 <listPattern>
 <finalPattern>
 <Noun>first</Noun>
 <Noun>any</Noun>
 </finalPattern>
 <ignorePattern/>
 </listPattern>
 <Noun>pair</Noun>
</defineExpr>

in Java:

Object specimen = pair;
if (!isList(specimen)) { throw(...); }
Integer len = E.toInteger(E.call(specimen, "size"));
if (len.intValue() != 2) { throw(...); }
final Object first = E.call(specimen, "get", 0);
# we hope to optimize this out this last useless call
E.call(specimen, "get", 1);

x

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