Patterns (ref)

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Contents 1 Patterns 2 Constructor pattern 3 Throw-away pattern 4 Record pattern 5 As binding 6 Type check pattern 7 Tuple pattern 8 List constructor pattern 9 List literal pattern 10 Literal pattern 11 Type enforcement in pattern

Patterns

Patterns are a form of accessing data structures, especially trees. Patterns can match values. A definition of the term to match is given with each pattern construct. However, the main idea behind patterns is that they match values that look like them.

Pattern are used in match expression and value definitions.

Constructor pattern

The identifier should refer to the name of variant option. This pattern matches a value if it is a specified variant option, and sub-pattern matches variant option payload.

Throw-away pattern

This pattern matches any value.

Record pattern

This pattern matches a value of a class, that has all specified fields (this is checked statically), and a value of each field matches respective pattern. This pattern currently cannot be used to match a value of a variant.

As binding

This pattern matches the same value as an enclosed pattern does. However, in addition the value matched by the enclosed pattern is bound to a specified variable, which can be used in when guard or match body.

Type check pattern

This pattern matches a value if it possesses the given type. In addition, the value matched is bound to a specified variable, which gets the given type.

This pattern can be used both for checking the type and hinting the type checker (if the value is statically known to always have given type, compiler issues a warning and no runtime checks are performed).

Tuple pattern

This pattern matches a tuple with specified contents (each tuple member is matched be respective pattern).

In addition, when a tuple pattern is seen, where a record pattern would be otherwise expected -- the tuple pattern is transformed to record pattern by adding field identifiers in order they appear in the definition of the given class. A tuple pattern transformed to a record pattern cannot match fields inherited from the base class.

List constructor pattern

Equivalent to:

list.Cons (pattern, pattern)

List literal pattern

Equivalent to:

list.Cons (pattern1, list.Cons (pattern2, ... list.Cons (pattern2, list.Nil) ... ))

Literal pattern

This pattern matches a specified constant value.

Type enforcement in pattern

This construct is used to statically enforce type of subpattern to given type. If a matched expression might not have such a type, compiler issues an error. It is used mainly for hinting type inference engine and improving code clarity.