Performance of a program written in dynamic languages is improved through the use of a compiler that provides type inference for methods having a user-defined element. The user-defined element may be an input in a user-defined type. Though, the user-defined element may reflect that the method is user-defined. Type inference may be performed based on a user-defined mapping, relating input types to output types for one or more methods. The mapping may be specified as a data table provided to the compiler or as one or more functions that register with the compiler such that, as the compiler processes portions of a source program and detects a method with a user-defined element, the compiler may access the mapping and infer the type of an output of the method. The inferred type may then be used to optimize code dependent on that output.