3.4.3 User-supplied field access function declarations

Type and mode declarations for compiler-generated field access functions for fields of constructors local to a module may be placed in the interface section of the module. The user-supplied declarations will be used instead of any automatically generated declarations. This allows the implementation of a type to be hidden while still allowing client modules to use record syntax to manipulate values of the type. Supplying a type declaration and a single mode declaration also allows higher-order terms to be created from a field access function without using explicit lambda expressions.

If a field occurs in the interface section of a module, then any declaration for a field access function for that field must also occur in the interface section.

If there are multiple fields with the same label in the same module, only one of those fields can have user-supplied declarations for its selection function. Similarly, only one of those fields can have user-supplied declarations for its update function.

Declarations and clauses for field access functions can also be supplied for fields which are not a part of any type. This is useful when the data structures of a program change so that a value which was previously stored as part of a type is now computed each time it is requested. It also allows record syntax to be used for type class methods.

User-declared field access functions may take extra arguments. For example, the Mercury standard library module map contains the following functions:

:- func elem(K, map(K, V)) = V is semidet.
:- func 'elem :='(K, map(K, V), V) = map(K, V).

Field access syntax may be used at the top-level of func and mode declarations and in the head of clauses. For instance:

:- func map(K, V) ^ elem(K) = V.
:- mode in        ^ in      = out is semidet.
Map ^ elem(Key) = map.lookup(Map, Key).

:- func (map(K, V) ^ elem(K) := V)  = V.
:- mode (in        ^ in      := in) = out is semidet.
(Map ^ elem(Key) := Value) = map.set(Map, Key, Value).

The Mercury standard library modules array and bt_array define similar functions.