[m-users.] string_writer / stream and writing strings with %s in them

[Sean Charles (emacstheviking)]

I am looking for some insights / advice on the best way to handle what has temporarily become something of a recusrsive horror story this afternoon! In my transpiler, there may or may not be strings containing C style printf() format controls. This caught me out

Uncaught Mercury exception:
Software Error: string.format: The first conversion specifier, which uses specifier character `s', is missing its input.

once so I implemented:

    % C_String :: Escapes any % with %% in the string.
    %
:- func c_string(ps::in) = (string::out).

c_string(ps(_,X)) = string.format("""%s""", [s(c_string_(X))]).

    % TODO: is this a typeclasss ie.  things to be escaped ??
:- func c_string_(string::in) = (string::out).



Which works but....as I tested my way through, the problem is compounded by the recursive nature of the rendering stage, resulting in strings like this:

   printf("Hello %%%%%s", "World");

caused by each successive handler making sure it escapes strings before returning up the stack. 
When I print the code out I am using string.format() and of course that blew up mightily until I started using %%s in --my-- strings of source code.

I currently use stream.string_writer.format() for generating the final output.

I wondered what strategy the Mercury compiler uses as it must have addressed the same issue at some point? I've tried reading the code to no avail!  I am rethinking through my approach, so far it has all worked great but then.. strings with %s and any other string escape for the C grade is going to have the same issues.

Thanks,
Sean.

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