Strongly-typed programming language

In computing, strongly-typed, when applied to a programming language, is used to describe how the language handles datatypes. "Strongly-typed" may have one of several incompatible meanings, depending on context. A programming language that is not strongly-typed is called weakly-typed. Some of the meanings that have been ascribed to "strongly-typed" include:

1. A language is strongly typed if type annotations are associated with variable names, rather than with values. If types are attached to values, it is weakly typed.
2. A language is strongly typed if it contains compile-time checks for type constraint violations. If all checking is deferred to run time, it is weakly typed.
3. A language is strongly typed if it contains compile or run-time checks for type constraint violations. If no checking is done, it is weakly typed.
4. A language is strongly typed if conversions between different types are forbidden. If such conversions are allowed, it is weakly typed.
5. A language is strongly typed if conversions between different types must be indicated explicitly. If implicit conversions are performed, it is weakly typed.
6. A language is strongly typed if there is no language-level way to disable or evade the type system. If there are casts or other type-evasive mechansisms, it is weakly typed.
7. A language is strongly typed if it has a complex, fine-grained type system with compound types. If it has only a few types, or only scalar types, it is weakly typed.
8. A language is strongly typed if the type of its data objects is fixed and does not vary over the lifetime of the object. If the type of a datum can change, the language is weakly typed.

Note that some of these definitions are contradictory, while others are merely orthogonal.

Because there is no generally-agreed meaning for the phrase "strongly-typed language", it is possible to find authoritative statements that many languages both are and are not strongly-typed. For example, under definitions 1, 7, and 8, the C language is strongly typed; under 4, 5, and 6 it is weakly typed - with definitions 2 and 3 open for further debate since C does performs type checks for compound types but not scalar and arrays types. Accordingly, it is easy to find people who will claim that C is a "strongly-typed language" and others who will claim that it is a "weakly-typed language".

Programming language expert Benjamin C. Pierce has said:

I spent a few weeks . . . trying to sort out the terminology of "strongly typed," "statically typed," "safe," etc., and found it amazingly difficult. . . . The usage of these terms is so various as to render them almost useless.

For a more thorough discussion of typing issues, see the article on datatypes and its related topics.