# Language Extensions¶

## Type Providers¶

Idris type providers are a way to get the type system to reflect observations about the world outside of Idris. Similarly to F# type providers, they cause effectful computations to run during type checking, returning information that the type checker can use when checking the rest of the program. While F# type providers are based on code generation, Idris type providers use only the ordinary execution semantics of Idris to generate the information.

A type provider is simply a term of type IO (Provider t), where Provider is a data type with constructors for a successful result and an error. The type t can be either Type (the type of types) or a concrete type. Then, a type provider p is invoked using the syntax %provide (x : t) with p. When the type checker encounters this line, the IO action p is executed. Then, the resulting term is extracted from the IO monad. If it is Provide y for some y : t, then x is bound to y for the remainder of typechecking and in the compiled code. If execution fails, a generic error is reported and type checking terminates. If the resulting term is Error e for some string e, then type checking fails and the error e is reported to the user.

Example Idris type providers can be seen at this repository. More detailed descriptions are available in David Christiansen’s WGP ‘13 paper and M.Sc. thesis.