[fpc-pascal] private type and type compatibility

Xiangrong Fang xrfang at gmail.com
Thu Oct 31 02:45:49 CET 2013

2013/10/30 Jonas Maebe <jonas.maebe at elis.ugent.be>

> This is not equivalent. A private type declaration in a class adds a new
> identifier that is visible inside that class. You then use it, still in
> that class, to declare the return type of a function. Next, in a scope
> where that type identifier is no longer visible, you call the function.
> My example is a complete match to that scenario as far as identifier
> visibility is concerned (you use a type in a scope where it is visible to
> declare a function return type, and then call the function in a scope where
> it is not visible). In your example, the type is not visible in the place
> where the function is declared but only where it is defined
> ​.

​This is logically WRONG. Because to the machine, any function return value
can be seen as an array of bytes, for example, a pointer is array[0..3] of
Byte on a 32-bit machine.  The purpose of type system is to explain what
these bytes stands for. So, if a type is out-of-scope, how do you interpret
the data?​

The current "delphi compatible" implementation IS using the type
information to compile the program, i.e. although it is not visible, it is
indeed used by the compile, which, in my opinion, violates visibility rules.

Standing on your view point, if a type is no longer visible, but a variable
(function return value) of that type is in current scope, and understood by
the program, this means, this value itself carries type information!  Is is
kind of meta data available in Pascal? If so, I think RTTI should work for
ANY kind of primitive data types.

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