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scala.tools.nsc.transform.patmat

PatternExpander

trait PatternExpander[Pattern, Type] extends AnyRef

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.

Source
PatternExpander.scala
Linear Supertypes
AnyRef, Any
Known Subclasses
ScalacPatternExpander, alignPatterns
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Type Members

  1. final case class Aligned (patterns: Patterns, extractor: Extractor) extends Product with Serializable

    If elementArity is...

    If elementArity is... 0: A perfect match between extractor and the fixed patterns. If there is a star pattern it will match any sequence. > 0: There are more patterns than products. There will have to be a sequence which can populate at least <elementArity> patterns. < 0: There are more products than patterns: compile time error.

  2. final case class Extractor (whole: Type, fixed: List[Type], repeated: Repeated, typeOfSinglePattern: Type) extends Product with Serializable

    An 'extractor' can be a case class or an unapply or unapplySeq method.

    An 'extractor' can be a case class or an unapply or unapplySeq method. Decoding what it is that they extract takes place before we arrive here, so that this class can concentrate only on the relationship between patterns and types.

    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.

    In other words, this case class and unapply are encoded the same:

    case class Foo(x: Int, y: Int, zs: Char*) def unapplySeq(x: Foo): Option[(Int, Int, Seq[Char])]

    Both are Extractor(Foo, Int :: Int :: Nil, Repeated(Seq[Char], Char, Char*))

    whole

    The type in its unextracted form

    fixed

    The non-sequence types which are extracted

    repeated

    The sequence type which is extracted

  3. final case class Patterns (fixed: List[Pattern], star: Pattern) extends Product with Serializable
  4. sealed case class Repeated (sequenceType: Type, elementType: Type, repeatedType: Type) extends Product with Serializable

    It's not optimal that we're carrying both sequence and repeated type here, but the implementation requires more unraveling before it can be avoided.

    It's not optimal that we're carrying both sequence and repeated type here, but the implementation requires more unraveling before it can be avoided.

    sequenceType is Seq[T], elementType is T, repeatedType is T*.

  5. final case class TypedPat (pat: Pattern, tpe: Type) extends Product with Serializable

Abstract Value Members

  1. abstract def NoPattern: Pattern

    You'll note we're not inside the cake.

    You'll note we're not inside the cake. "Pattern" and "Type" are arbitrary types here, and NoPattern and NoType arbitrary values.

  2. abstract def NoType: Type

Concrete Value Members

  1. object NoRepeated extends Repeated