HP Fortran Programmer's Guide (B3908-90031; September 2011)

Controlling data storage
Disabling implicit typing
Chapter 3 103
Disabling implicit typing
By default, HP Fortran uses implicit typing to determine the type of a variable or function that has not
been declared with a type declaration statement. That is, the type of an undeclared entity is determined by
the first letter of its name: if the letter is in the range I - N, the entity is of type integer; otherwise, it is of
type real.
Although implicit typing is mandated by the Standard, its use can become a source of runtime bugs because
implicit typing allows the inadvertent use of undeclared variables or functions. For the sake of illustration,
consider a program that calls a nonintrinsic library function named foo. Assume that:
• The default typing rules are in effect.
• foo returns an integer.
• The programmer has not declared the return type of foo and has assigned its return value to a variable
of type real.
Experience has shown that this is not an unlikely scenario and that it can produce unexpected results.
The Standard provides the IMPLICIT NONE statement to override implicit typing. But the IMPLICIT
NONE statement is limited in scope to the program unit in which it appears. To force explicit typing for all
files specified on the command line, use the +implicit_none option. This option disables implicit
typing; that is, all variables, arrays, named constants, function subprograms, ENTRY names, and statement
functions (but not intrinsic functions) must be explicitly declared.
Using this option is equivalent to specifying IMPLICIT NONE for each program unit in each file specified
on the f90 command line. However, the +implicit_none option does not override any IMPLICIT
statements in the source file. The HP Fortran Programmer's Reference describes the implicit typing rules,
the IMPLICIT NONE statement, and the +implicit_none option.