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6.6  Patterns

pattern::= value-name  
  pattern as  value-name  
  ( pattern )  
  ( pattern :  typexpr )  
  pattern |  pattern  
  constr  pattern  
  `tag-name  pattern  
  pattern  { , pattern }+  
  { field  [: typexpr=  pattern { ; field  [: typexpr=  pattern }  [ ; ] }  
  [ pattern  { ; pattern }  [ ; ] ]  
  pattern ::  pattern  
  [| pattern  { ; pattern }  [ ; ] |]  
  char-literal ..  char-literal

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

lazy (see section 7.3)
Constructor application, Tag applicationright

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 be bound several times by 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 value-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 typexpr.

“Or” patterns

The pattern pattern1 |  pattern2 represents the logical “or” of the two patterns pattern1 and pattern2. A value matches pattern1 |  pattern2 if it matches pattern1 or 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 ,,  patternn ) matches all variants whose constructor is equal to constr, and whose arguments match pattern1 …  patternn. It is a type error if n is not the number of arguments expected by the constructor.

The pattern constr _ matches all variants whose constructor is constr.

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

The pattern [ pattern1 ;;  patternn ] matches lists of length n whose elements match pattern1patternn, respectively. This pattern behaves like pattern1 ::::  patternn :: [].

Polymorphic variant patterns

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

Polymorphic variant abbreviation patterns

If the type [('a,'b,)] typeconstr = [ ` tag-name1  typexpr1 || ` tag-namen  typexprn] is defined, then the pattern #typeconstr is a shorthand for the following or-pattern: ( `tag-name1(_ :  typexpr1) || ` tag-namen(_ :  typexprn)). It matches all values of type [< typeconstr ].

Tuple patterns

The pattern pattern1 ,,  patternn matches n-tuples whose components match the patterns pattern1 through patternn. That is, the pattern matches the tuple values (v1, …, vn) such that patterni matches vi for i = 1,… , n.

Record patterns

The pattern { field1 =  pattern1 ;;  fieldn =  patternn } matches records that define at least the fields field1 through fieldn, and such that the value associated to fieldi matches the pattern patterni, for i = 1,… , n. The record value can define more fields than field1fieldn; the values associated to these extra fields are not taken into account for matching. Optional type constraints can be added field by field with { field1 :  typexpr1 =  pattern1 ;; fieldn :  typexprn =  patternn } to force the type of fieldk to be compatible with typexprk.

Array patterns

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

Range patterns

The pattern ' c ' .. ' d ' is a shorthand for the pattern

' c ' | ' c1 ' | ' c2 ' || ' cn ' | ' d '

where c1, c2, …, cn are the characters that occur between c and d in the ASCII character set. For instance, the pattern '0'..'9' matches all characters that are digits.

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