Data.Pattern.Base

The main types used in the implementation of first-class patterns: Pattern and Clause.

Patterns

newtype Pattern vars a

The pattern type. A value of type Pattern vars a is a pattern which matches values of type a and binds variables with types given by the type-list vars. For example, something of type

Pattern (a :*: c :*: Nil) (a,b,c)

is a pattern which matches against a triple and binds values of types a and c. (A pattern of this type can be constructed as tup3 var __ var.)

Many "normal" patterns can be conveniently defined using mk0, mk1, mk2, and so on.

Constructors

Pattern

Fields

runPattern :: a -> Maybe (Tuple vars)

Clauses

data Clause a r

Pattern-match clauses. Typically something of the form

pattern ->> function

where the function takes one argument for each variable bound by the pattern.

Clauses can be constructed with (->>), run with tryMatch, and manipulated by the Monad and MonadPlus instances. In particular, the (<|>) operator from the Alternative class is the way to list multiple cases in a pattern.

Instances

Alternative (Clause a)

Monad (Clause a)

Functor (Clause a)

MonadPlus (Clause a)

Applicative (Clause a)

runClause :: Clause a r -> ReaderT a Maybe r

Extract the underlying computation constituting a Clause. This function is not intended to be used directly; instead, see match, tryMatch, mmatch, and elim from Data.Pattern.Common.

(->>) :: Pattern vars a -> Fun vars r -> Clause a r infix 4

Construct a Clause from a pattern and a function which takes one argument for each variable bound by the pattern. For example,

pair __ nothing     ->> 3
pair var nothing    ->> \x -> x + 3
pair var (just var) ->> \x y -> x + y + 3

(<|>) :: Alternative f => forall a. f a -> f a -> f a

An associative binary operation

Internals

module Data.Pattern.Base.TypeList

module Data.Pattern.Base.Tuple