🚀 go-pugleaf

In article <243@gould.doc.ic.ac.uk> cdsm@doc.ic.ac.uk (Chris Moss) writes:
>... I personally think "repeat" should
>be left out of the standard (there's no penalty overhead in not having it
>built-in these days anyway). Don't other people have my problem?

	Unfortunately, there is a penalty when it is not built-in.
	If repeat/0 is coded as

	repeat.
	repeat :- repeat.

	it creates a new choice point each time the system backtracks
	to it, however this is not the main point, I'm sure there
	are clever compilers around that could get by with it.
	The problem concerns the Byrd box model for debugging and it was
	pointed to me by Thomas Graf:
	if repeat/0 is built-in, it is called only once (it enters the CALL
	port and leaves through the EXIT port), on backtracking it enters
	by REDO and leaves by EXIT. When its choice point is cut,
	it just exits or fails.

	The situation is different with the above Prolog coding:
	on each backtracking to repeat/0 a new call is made, i.e. a new box
	and hence there are two more ports to trace. Look at this
	script from SICStus Prolog:

| ?- [user].
| repeat1.
| repeat1 :- repeat1.
| ^D
yes
| ?- trace, repeat1, fail.
 
 The debugger will first creep -- showing everything (trace).
 1  1  Call: repeat1 ? 
 1  1  Exit: repeat1 ? 
 2  1  Call: fail ? 
 2  1  Fail: fail ? 
 1  1  Redo: repeat1 ? 
 2  2  Call: repeat1 ? 
 2  2  Exit: repeat1 ? 
 1  1  Exit: repeat1 ? 
 3  1  Call: fail ? 
 3  1  Fail: fail ? 
 1  1  Redo: repeat1 ? 
 2  2  Redo: repeat1 ? 
 3  3  Call: repeat1 ? 
 3  3  Exit: repeat1 ? 
 2  2  Exit: repeat1 ? 
 1  1  Exit: repeat1 ? 
 4  1  Call: fail ? 

	etc., while the built-in repeat/0 behaves normally:

| ?- repeat, fail.
1  1  Call: repeat ? 
1  1  Exit: repeat ? 
2  1  Call: fail ? 
2  1  Fail: fail ? 
1  1  Redo: repeat ? 
1  1  Exit: repeat ? 
2  1  Call: fail ? 
2  1  Fail: fail ? 
1  1  Redo: repeat ? 
1  1  Exit: repeat ? 
2  1  Call: fail ? 

	You can say that it is possible to skip over the multiple
	'repeat' ports, but the point is, that the stack space for them
	is needed even when they are not printed - you cannot
	run such a repeat/0 forever, eventually it is going to overflow
	some stack.
	On the other hand, we could ask whether the box model is right
	in this case - after all it does not bring any new information
	by repeating all these ports.

	There is no reasonable argument to force people to use
	tail-recursive loops instead of repeat-fail loops;
	if I'm using temporary structures and I know that with the
	recursive loops they are going to be garbage collected whereas
	with repeat-fail loops they are just popped, I will always
	prefer the latter and the standard should support me
	by providing a built-in repeat/0, since its full functionality
	cannot be provided by other means.

---

Another point I want to make concerns the -1 returned by get0/1
at eof: several people have claimed that it is portable
and that it cannot be confused with any character, however
it is *not* portable, since it relies on the fact that
no valid character can be confused with -1. If characters are
represented as strings of length 1, then -1 has a different type
and so there is no confusion, but the eof value should have the same type
(if nothing else then because of indexing). If characters are integers,
taking -1 implies that no character can have the code 2^n - 1
(n being the number of bits on which the character is stored)
which is not necessarily true - you can use 7 bits for ASCII, 16 bits for
Kanji and anything else on any number of bits. Only if we waste
enough space we can guarantee that -1 will be different.
A standard that forces you to waste space would really not be good.

--Micha