A function expression is an expression that returns a single value that is the result of invoking a procedure. The procedure can be declared in the program, or it can be an intrinsic procedure.

<function expression>

──┬─<arithmetic function designator>─┬─────────────────────────────────┤
  ├─<Boolean function designator>────┤
  ├─<complex function designator>────┤
  ├─<pointer function designator>────┤
  └─<string function designator>─────┘

The two kinds of functions are predefined, or intrinsic, functions which are part of the ALGOL language, and programmer-defined functions, which are typed procedures that are declared in the program.

The intrinsic functions are described under Intrinsic Functions.