Patterns

6.6 Patterns

`pattern`	::=	`value-name`
	∣	`_`
	∣	`constant`
	∣	`pattern` `as` `value-name`
	∣	`(` `pattern` `)`
	∣	`(` `pattern` `:` `typexpr` `)`
	∣	`pattern` `\|` `pattern`
	∣	`constr` `pattern`
	∣	``tag-name` `pattern`
	∣	`#typeconstr-name`
	∣	`pattern` { `,` `pattern` }
	∣	`{` `field` `=` `pattern` { `;` `field` `=` `pattern` } `}`
	∣	`[` `pattern` { `;` `pattern` } `]`
	∣	`pattern` `::` `pattern`
	∣	`[\|` `pattern` { `;` `pattern` } `\|]`

The table below shows the relative precedences and associativity of operators and non-closed pattern constructions. The constructions with higher precedences come first.

Operator	Associativity
Constructor application	–
`::`	right
`,`	–
`\|`	left
`as`	–

Patterns are templates that allow selecting data structures of a given shape, and binding identifiers to components of the data structure. This selection operation is called pattern matching; its outcome is either “this value does not match this pattern”, or “this value matches this pattern, resulting in the following bindings of names to values”.

Variable patterns

A pattern that consists in a value name matches any value, binding the name to the value. The pattern _ also matches any value, but does not bind any name.

Patterns are linear: a variable cannot appear several times in a given pattern. In particular, there is no way to test for equality between two parts of a data structure using only a pattern (but when guards can be used for this purpose).

Constant patterns

A pattern consisting in a constant matches the values that are equal to this constant.

Alias patterns

The pattern pattern₁ as value-name matches the same values as pattern₁. If the matching against pattern₁ is successful, the name name is bound to the matched value, in addition to the bindings performed by the matching against pattern₁.

Parenthesized patterns

The pattern ( pattern₁ ) matches the same values as pattern₁. A type constraint can appear in a parenthesized pattern, as in ( pattern₁ : typexpr ). This constraint forces the type of pattern₁ to be compatible with type.

“Or” patterns

The pattern pattern₁ | pattern₂ represents the logical “or” of the two patterns pattern₁ and pattern₂. A value matches pattern₁ | pattern₂ either if it matches pattern₁ or if it matches pattern₂. The two sub-patterns pattern₁ and pattern₂ must bind exactly the same identifiers to values having the same types. Matching is performed from left to right. More precisely, in case some value v matches pattern₁ | pattern₂, the bindings performed are those of pattern₁ when v matches pattern₁. Otherwise, value v matches pattern₂ whose bindings are performed.

Variant patterns

The pattern constr pattern₁ matches all variants whose constructor is equal to constr, and whose argument matches pattern₁.

The pattern pattern₁ :: pattern₂ matches non-empty lists whose heads match pattern₁, and whose tails match pattern₂.

The pattern [ pattern₁ ; … ; pattern_n ] matches lists of length n whose elements match pattern₁ ...pattern_n, respectively. This pattern behaves like pattern₁ :: … :: pattern_n :: [].

Polymorphic variant patterns

The pattern `tag-name pattern₁ matches all polymorphic variants whose tag is equal to tag-name, and whose argument matches pattern₁.

Variant abbreviation patterns

If the type [('a,'b,...)] typeconstr = [`tag₁ t₁ | … | `tag_n t_n] is defined, then the pattern #typeconstr is a shorthand for the or-pattern (`tag₁(_ : t₁) | … | `tag_n(_ : t_n)). It matches all values of type #typeconstr.

Tuple patterns

The pattern pattern₁ , … , pattern_n matches n-tuples whose components match the patterns pattern₁ through pattern_n. That is, the pattern matches the tuple values (v₁, …, v_n) such that pattern_i matches v_i for i = 1,… , n.

Record patterns

The pattern { field₁ = pattern₁ ; … ; field_n = pattern_n } matches records that define at least the fields field₁ through field_n, and such that the value associated to field_i matches the pattern pattern_i, for i = 1,… , n. The record value can define more fields than field₁ ...field_n; the values associated to these extra fields are not taken into account for matching.

Array patterns

The pattern [| pattern₁ ; … ; pattern_n |] matches arrays of length n such that the i-th array element matches the pattern pattern_i, for i = 1,… , n.