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Duck typing is similar to, but distinct from, structural typing.Structural typing is a static typing system that determines type compatibility and equivalence by a type's structure, whereas duck typing is dynamic and determines type compatibility by only that part of a type's structure that is accessed during runtime.
Boo is an object-oriented, statically typed, general-purpose programming language that seeks to make use of the Common Language Infrastructure's support for Unicode, internationalization, and web applications, while using a Python-inspired syntax [2] and a special focus on language and compiler extensibility.
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.
The process of verifying and enforcing the constraints of types—type checking—may occur at compile time (a static check) or at run-time (a dynamic check). If a language specification requires its typing rules strongly, more or less allowing only those automatic type conversions that do not lose information, one can refer to the process as strongly typed; if not, as weakly typed.
Dynamic polymorphism is more flexible but slower—for example, dynamic polymorphism allows duck typing, and a dynamically linked library may operate on objects without knowing their full type. Static polymorphism typically occurs in ad hoc polymorphism and parametric polymorphism, whereas dynamic polymorphism is usual for subtype polymorphism.
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Rewriting the above "hello world!" example to instead be a sequence of two netstrings, itself encoded as a single netstring, gives the following: 17: 5: hello, 6: world!,, Parsing such a nested netstring is an example of duck typing, since the contained string ("5:hello,6:world!,") is both a string and a sequence of netstrings. Its effective ...
For example, if a language allows new types to be declared, a CFG cannot predict the names of such types nor the way in which they should be used. Even if a language has a predefined set of types, enforcing proper usage usually requires some context. Another example is duck typing, where the type of an element can change depending on context.
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