Packages

t

scala.tools.nsc.transform.patmat

PatternExpansion

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
Linear Supertypes
AnyRef, Any
Known Subclasses
patmat, PatternMatching
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Type Members

  1. class ExtractorAlignment extends ExtractorSubPatternAlignment
  2. trait ExtractorSubPatternAlignment extends AnyRef

    The arities we can derive looking only at the subpatterns (the args of the unapply node)

Abstract Value Members

  1. abstract val global: Global

Concrete Value Members

  1. def unapplyFormals(fun: Global.Tree, args: List[Global.Tree])(context: Context): List[Global.Type]