6.4 Declaring the Type of Functions

In ugTypesDeclare we discussed how to declare a variable to restrict the kind of values that can be assigned to it. In this section we show how to declare a variable that refers to function objects.

A function is an object of type

{\sf Source → Type}

where Source and Target can be any type. A common type for Source is

Tuple(, ..., ), usually written (, ..., ), to indicate a function of n arguments.

If g takes an Integer, a Float and another Integer, and returns a String, the declaration is written:

g: (Integer,Float,Integer) -> String

_{Type: Void}

The types need not be written fully; using abbreviations, the above declaration is:

g: (INT,FLOAT,INT) -> STRING

_{Type: Void}

It is possible for a function to take no arguments. If h takes no arguments but returns a Polynomial Integer, any of the following declarations is acceptable.

h: () -> POLY INT

_{Type: Void}

h: () -> Polynomial INT

_{Type: Void}

h: () -> POLY Integer

_{Type: Void}

Functions can also be declared when they are being defined. The syntax for combined declaration/definition is:

functionName(parm1: parmType1, ..., parmN: parmTypeN): functionReturnType

The following definition fragments show how this can be done for the functions g and h above.

g(arg1: INT, arg2: FLOAT, arg3: INT): STRING == ...
h(): POLY INT == ...

A current restriction on function declarations is that they must involve fully specified types (that is, cannot include modes involving explicit or implicit ?). For more information on declaring things in general, see ugTypesDeclare .