Question

is it possible to read and write from/to file using only 1 assign()?

I mean something like:

assign(fl, 'file.txt');
reset(fl) 
// I know, that when I use reset - I have only read permission, 
// but is there any other option to read and write opening the file only once?
read(fl, variable);
write(fl, 'asdf);
close(fl);

Answer 1

Reset does not give you only read access; the default is read/write (FileMode = 2), at least in Delphi’s Pascal, and FP is pretty compatible with it.

From the help topic on FileMode (emphasis mine):

Indicates the access mode on typed and untyped files opened by the Reset procedure.

In Delphi code, FileMode determines the access mode to use when typed and untyped files (not text files) are opened using the Reset procedure.

The file open modes that can be assigned to the FileMode variable are defined in the Constants list on System.SysUtils. The default FileMode is 2 (Read/Write access). Assigning another value to FileMode causes all subsequent Resets to use that mode.