Assignment compatibility

This section discusses the assignment issues surrounding assignment compatibility of elementary types (INTEGER, CARDINAL, REAL and CHAR for example). The information here is found in more detail in the Modula-2 ISO standard BS ISO/IEC 10514-1:1996 page 122.

Assignment compatibility exists between the same sized elementary types.

Same type family of different sizes are also compatible as long as the MAX( type ) and MIN( type ) is known. So for example this includes the INTEGER family, CARDINAL family and the REAL family.

The reason for this is that when the assignment is performed the compiler will check to see that the expression (on the right of the :=) lies within the range of the designator type (on the left hand side of the :=). Thus these ordinal types can be assignment compatible. However it does mean that WORD32 is not compatible with WORD16 as WORD32 does not have a minimum or maximum value and therefore cannot be checked. The compiler does not know which of the two bytes from WORD32 should be copied into WORD16 and which two should be ignored. Currently the types BITSET8, BITSET16 and BITSET32 are assignment incompatible. However this restriction maybe lifted when further runtime checking is achieved.

Modula-2 does allow INTEGER to be assignment compatible with WORD as they are the same size. Likewise GNU Modula-2 allows INTEGER16 to be compatible with WORD16 and the same for the other fixed sized types and their sized equivalent in either WORD n, BYTE or LOC types. However it prohibits assignment between WORD and WORD32 even though on many systems these sizes will be the same. The reasoning behind this rule is that the extended fixed sized types are meant to be used by applications requiring fixed sized data types and it is more portable to forbid the bluring of the boundaries between fixed sized and machine dependant sized types.

Intemediate code runtime checking is always generated by the front end. However this intemediate code is only translated into actual code if the appropriate command line switches are specified. This allows the compiler to perform limited range checking at compile time. In the future it will allow the extensive GCC optimisations to propagate constant values through to the range checks which if they are found to exceed the type range will result in a compile time error message.