Targettype

Targettype is a term used in programming language theory and compiler design to denote the type that an expression is expected to have in a given context. It represents the destination type that the language’s type system uses when checking, inferring, or converting expressions.

In many languages, the targettype is determined by the surrounding code rather than being explicit in the expression itself. For example, the targettype can be the declared type of a variable receiving a value, the parameter type of a function call, or the return type of a function being assigned to. The concept is central to type inference and bidirectional type checking, where the context guides how an expression should be interpreted and which conversions are permissible.

The targettype influences conversions and casts. If an expression can be converted to multiple types, the targettype helps the compiler choose a safe or most natural conversion. Some languages allow implicit conversions toward the targettype, while others require an explicit cast or annotation when the conversion could change behavior or precision.

A common practical manifestation occurs with lambda expressions or anonymous functions, where the language infers parameter and return types from the targettype of the functional interface or expected function type at the call site. Similarly, in statically typed languages with type inference, the targettype enables concise code by reducing the need for explicit type annotations.

Overall, targettype is a foundational concept for understanding how expressions acquire their types within a program’s context, impacting readability, safety, and flexibility.