🚀 go-pugleaf

In article <brady-300992144101@brady.cs.tcd.ie>, brady@cs.tcd.ie (Michael Brady) writes:
> My memory, which could easily be wrong, was that if for read(Term), it read
> in the first term it came across, and failed if that term wasn't unifiable
> with Term. It would also report syntax errors...
> For get0(Char), it read in a character, and if the character didn't unify
> with Char, it would advance to the next character and try again, ad
> infinitum until the right character was read.
> Is my memory playing tricks on me?

Yes.

All the input commands in DEC-10 Prolog had the same behaviour: guessing
wrong about the output argument does not affect which side effects take
place, only whether the query succeeds or fails.

In a mix of Prolog and C:

	get0(Char) :-
		( feof(current_input) -> error
		; Char is getc(current_input),
		).

	get(Char) :-
		( feof(current_input) -> error
		; Temp is getc(current_input),
		  ( Temp =:= EOF ->
		    Char is 26	% in DEC-10 Prolog
		    Char is -1  % in just about every other Prolog since
		  ; isspace(Temp) ->
		    get(Char)
		  ; Char is Temp
		  )
		).

	skip(Char) :-
		C is Char,	% Char may be an expression
		repeat,		% repeat/0 is needed because
		    get0(C),	% get0(C) WON'T skip
		!.

	read(Term) :-
		repeat,
		    get_tokens(Tokens),
		    parse(Tokens, Result),
		!,
		Term = Result.

> The real question is what is the most desirable action in these cases. I
> agree with Lee about read/1, but how about get0/1 and get/1?

They should be compatible with DEC-10 Prolog, C Prolog, PDP-11 Prolog,
Prolog-X, SICStus Prolog, NU Prolog, Quintus Prolog, &c.  It would be best
if the default behaviour was to return -1 for EOF, but a DEC-10 compatibility
mode would be of use as at least once PC implementation still uses 26 for
EOF (because CP/M did, basically).
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