trait PatternExpansion extends AnyRef
An 'extractor' can be a case class or an unapply or unapplySeq method.
In a case class, the class is the unextracted type and the fixed and repeated types are derived from its constructor parameters.
In an unapply, this is reversed: the parameter to the unapply is the unextracted type, and the other types are derived based on the return type of the unapply method.
An extractor returns: F1, F2, ..., Fi, opt[Seq[E] or E*] A case matches: P1, P2, ..., Pj, opt[Seq[E]] Put together: P1/F1, P2/F2, ... Pi/Fi, Pi+1/E, Pi+2/E, ... Pj/E, opt[Seq[E]]
Here Pm/Fi is the last pattern to match the fixed arity section.
productArity: the value of i, i.e. the number of non-sequence types in the extractor nonStarArity: the value of j, i.e. the number of non-star patterns in the case definition elementArity: j - i, i.e. the number of non-star patterns which must match sequence elements starArity: 1 or 0 based on whether there is a star (sequence-absorbing) pattern totalArity: nonStarArity + starArity, i.e. the number of patterns in the case definition
Note that productArity is a function only of the extractor, and nonStar/star/totalArity are all functions of the patterns. The key value for aligning and typing the patterns is elementArity, as it is derived from both sets of information.
If elementArity is...
- zero: A perfect match between extractor and the fixed patterns. If there is a star pattern it will match any sequence.
- positive: There are more patterns than products. There will have to be a
sequence which can populate at least
elementArity
patterns. - negative: There are more products than patterns: compile time error.
- Source
- PatternExpansion.scala
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Type Members
- class ExtractorAlignment extends ExtractorSubPatternAlignment
-
trait
ExtractorSubPatternAlignment extends AnyRef
The arities we can derive looking only at the subpatterns (the args of the unapply node)
Concrete Value Members
- def unapplyFormals(fun: Global.Tree, args: List[Global.Tree])(context: Context): List[Global.Type]
The Scala compiler and reflection APIs.