pattern-matching.md

layout	title	category	tags	order
developer-doc	Pattern Matching	types	types pattern-matching	5

Pattern Matching

Pattern matching in Enso works similarly to as you would expect in various other functional languages. Typing information is always refined in the branches of a case expression, which interacts well with dependent typing and type-term unification.

The actionables for this section :

• How do we type pattern matching?
• Exactly how (and what) evidence is discharged during matching?
• How can we use bijective applications of the typeset operators to perform pattern matching?
• How do we extend this to structural matching in general on typesets?

• Positional Matching
• Type Matching
• Name Matching on Labels
• Naming Scrutinees

Positional Matching

Matching on the scrutinee by structure. This works both for atoms and typesets (for typesets it is a subsumption judgement).

type Vector a
  V2 x:a y:a
  V3 x:a y:a z:a

v = Vector.V3 x y z

case v of
  Vector.V3 x y z -> print x

Type Matching

Matching purely by the types involved, and not matching on structure.

case v of
  Vector.V3 -> print v.x

Name Matching on Labels

Matching on the labels defined within a type for both atoms and typesets, with renaming.

case v of
  Vector.V3 {x y} -> print x
  {x}             -> print x

Naming Scrutinees

Ascribing a name of a scrutinee is done using the standard typing judgement. This works due to the type-term unification present in Enso.

case _ of
  v : Vector.V3 -> print v,x