A structural type system (or property-based type system) is a major class of type system, in which type compatibility and equivalence are determined by the type's actual structure or definition, and not by other characteristics such as its name or place of declaration. Structural systems are used to determine if types are equivalent and whether a type is a subtype of another. It contrasts with nominative systems, where comparisons are based on the names of the types or explicit declarations, and duck typing, in which only the part of the structure accessed at runtime is checked for compatibility.
In structural typing, an element is considered to be compatible with another if, for each feature within the second element's type, there is a corresponding and identical feature in the first element's type. Some languages may differ on the details, such as whether the features must match in name. This definition is not symmetric, and includes subtype compatibility. Two types are considered to be identical if each is compatible with the other.
As an example, OCaml uses structural typing on methods for compatibility of object types. Go uses structural typing on methods to determine compatibility of a type with an interface. C++ template functions exhibit structural typing on type arguments. Haxe uses structural typing, although classes are not structurally subtyped.
In languages which support subtype polymorphism, a similar dichotomy can be formed based on how the subtype relationship is defined. One type is a subtype of another if and only if it contains all the features of the base type (or subtypes thereof); the subtype may contain additional features (such as members not present in the base type, or stronger invariants).
There is a distinction between structural substitution for inferred and non-inferred polymorphism. Some languages, such as Haskell, do not substitute structurally in the case where an expected type is declared (i.e. not inferred), e.g. only substitute for functions that are signature-based polymorphic via type inference. Then it is not possible to accidentally subtype a non-inferred type, although it may still be possible to provide an explicit conversion to a non-inferred type, which is invoked implicitly.
Structural subtyping is arguably more flexible than nominative subtyping, as it permits the creation of ad hoc types and protocols; in particular, it permits creation of a type which is a supertype of an existing type, without modifying the definition of the latter. However, this may not be desirable where the programmer wishes to create closed abstractions.
A pitfall of structural typing versus nominative typing is that two separately defined types intended for different purposes, but accidentally holding the same properties (e.g. both composed of a pair of integers), could be considered the same type by the type system, simply because they happen to have identical structure. One way this can be avoided is by creating one algebraic data type for each use.
In 1990, Cook, et al., proved that inheritance is not subtyping in structurally-typed OO languages.
Here the OCaml interactive runtime prints out the inferred type of the object for convenience. You can see that its type (< get_x : int; set_x : int -> unit >) is purely defined by its methods. In other words the type of x is defined by the method types "get_x : int" and "set_x : int -> unit" rather than by any name.
Let's define another object, which has the same methods and types of methods: