Patterns are used in combinators, in val declarations, and in clausal definitions of functions, to select values and bind variables to values. A pattern is given by a shape and a set of variables. A shape is either a tuple, a list, a record, a call, a literal value, or wildcard (written as _). If the shape describes a structured value (such as a tuple), its components may also be shapes. For example, the shape (_,3) describes all pairs whose second element is 3, and the pattern (x,3) binds x to the first element of all such pairs.

Note that a pattern may fail to match a value, if it does not have the same shape as that value. When this occurs, the unmatched value is simply discarded.

A pattern such as (x,y) may bind multiple variables. However, patterns are linear, meaning that a pattern may mention a variable name at most once. For example, (x,y,x) is not a valid pattern.

During typechecking, a pattern is matched against a type instead of a value. This match produces a typing context, which associates a type with each variable that occurs in the pattern.