Fortran: IMPLICIT NONE

The introduction of IMPLICIT NONE into the Fortran 90 standard ensured that there was now a method of invoking strict type declarations within the language while still retaining Fortran's legendary backwards compatibility. It is widely accepted that any program block written for Fortran 90 or above should include an IMPLICIT NONE statement.

PROGRAM foo
IMPLICIT NONE
INTEGER :: a = 1, b = 2, c = 3
REAL    :: i = 2.5, j = -15.7
PRINT *, 'I = ', i, ', A = ', a
END PROGRAM foo

While many earlier compilers shipped with a similar statement as an extension to the F77 standard, this was not universal and so a common workaround for truly portable code is to use "IMPLICIT LOGICAL (A-Z)" which obviously prevents assigning integers, reals or characters to undeclared variables.