3.4.1 Field selection

field(Term)

Each field label ‘field’ in a constructor causes generation of a field selection function ‘field/1’, which takes a data-term of the same type as the constructor and returns the value of the labelled field, failing if the top-level constructor of the argument is not the constructor containing the field.

If the declaration of the field is in the interface section of the module, the corresponding field selection function is also exported from the module.

By default, this function has no declared modes — the modes are inferred at each call to the function. However, the type and modes of this function may be explicitly declared, in which case it will have only the declared modes.

To create a higher-order term from a field selection function, an explicit lambda expression must be used, unless a single mode declaration is supplied for the field selection function. The reason for this is that normally, field access functions are implemented directly as unifications, without the code of a function being generated for them. The declaration acts as the request for the generation of that code.