datatype 'a 'b either = Left of 'a | Right of 'b ML, 1973. Pascal is only 3 years older, but much much sillier.
On Tue, Oct 30, 2012 at 11:39 PM, Cédric Beust ♔ <ced...@beust.com> wrote: > I hope you're not suggesting that I was contrasting the quality of a piece > of code that's forty years old with one written in a modern programming > language... > > -- > Cédric > > > > > On Tue, Oct 30, 2012 at 7:34 PM, Ricky Clarkson <ricky.clark...@gmail.com> > wrote: >> >> The difference being that your Pascal example has no notion of one >> side being the real side, hence it being useful for subverting the >> type system, and an Either has that defined and doesn't allow any >> subversion, but lets you abstract over it being one thing or the >> other. In C it can be used for 'evil' too but a common approach is to >> have a tag saying what the type really is. >> >> On Tue, Oct 30, 2012 at 11:25 PM, Cédric Beust ♔ <ced...@beust.com> wrote: >> > Bouncing off another topic mentioned in the latest podcast: union types. >> > Yes, C had them but the first time I was exposed to them was in Pascal, >> > and >> > the discussion on the podcast reminded me of an awesome hack that blew >> > my >> > mind a very long time ago. >> > >> > Pascal was well known to be very strict and safe. Among other things, it >> > didn't let you access the memory directly, which was a big deal in the 8 >> > bit >> > era where memory protection was a distant dream and PEEK and POKE were >> > how >> > you wrote games. >> > >> > And then, one day, somebody found a way to address the memory using >> > standard >> > Pascal. Here is the trick (from memory, so it's probably not quite >> > correct): >> > >> > type >> > b = record >> > x : array[1..65536] of ^integer; >> > y : integer; >> > end; >> > >> > This declares a union type that is made of either an array of 65k >> > pointers >> > to integers or a single integer. Then you initialize this record in its >> > "x" >> > side with the memory address you want to peek and you access it by using >> > the >> > "x" side of the record. >> > >> > It took me months to understand what this code did, but what a >> > revelation it >> > was when it finally clicked... >> > >> > By the way, these types are formally known as "sum types" (Either is one >> > of >> > them). >> > >> > -- >> > Cédric >> > >> > >> > -- >> > You received this message because you are subscribed to the Google >> > Groups >> > "Java Posse" group. >> > To post to this group, send email to javaposse@googlegroups.com. >> > To unsubscribe from this group, send email to >> > javaposse+unsubscr...@googlegroups.com. >> > For more options, visit this group at >> > http://groups.google.com/group/javaposse?hl=en. >> >> -- >> You received this message because you are subscribed to the Google Groups >> "Java Posse" group. >> To post to this group, send email to javaposse@googlegroups.com. >> To unsubscribe from this group, send email to >> javaposse+unsubscr...@googlegroups.com. >> For more options, visit this group at >> http://groups.google.com/group/javaposse?hl=en. >> > > -- > You received this message because you are subscribed to the Google Groups > "Java Posse" group. > To post to this group, send email to javaposse@googlegroups.com. > To unsubscribe from this group, send email to > javaposse+unsubscr...@googlegroups.com. > For more options, visit this group at > http://groups.google.com/group/javaposse?hl=en. -- You received this message because you are subscribed to the Google Groups "Java Posse" group. To post to this group, send email to javaposse@googlegroups.com. To unsubscribe from this group, send email to javaposse+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/javaposse?hl=en.
