Path: utzoo!mnetor!uunet!husc6!bbn!rochester!PT.CS.CMU.EDU!sei!sei.cmu.edu!firth
From: firth@sei.cmu.edu (Robert Firth)
Newsgroups: comp.lang.pascal
Subject: Re: Pascal Enumerated I/O
Message-ID: <4294@aw.sei.cmu.edu>
Date: 22 Feb 88 14:24:48 GMT
References: <11903@brl-adm.ARPA> <673@cresswell.quintus.UUCP> <1077@csuna.UUCP>
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Reply-To: firth@bd.sei.cmu.edu.UUCP (Robert Firth)
Organization: Carnegie-Mellon University, SEI, Pgh, Pa
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In article <1077@csuna.UUCP> abcscnuk@csuna.UUCP (Naoto Kimura) writes:

>Ok, then go and implement a compiler that will allow input of enumerated types.
>Output I can see how it could be done, but input is another story.   The
>compiler would have to have something like yacc or lex built into it.

The Ada language requires both input and output of enumerated types.  It
is not really that hard.  Consider

	type Day = (Sunday, Monday, Tuesday, ...);

You build a string table in the order of the type values; simply index this
table by the binary ordinal to get the display form.

For input, you can use the same table if the number of values is small,
searching it linearly; otherwise, build a true lookup table with the text
values in alphabetical order, and search by binary chop.  Or you can get
really fancy, and use a hash table.

It so happens I wrote this part of an Ada runtime support package, and it
was less than a page of code for both translation procedures.  To answer
the usual silly questions

(a) the language is not case sensitive, so you force to uniform case on
    both input and output.

(b) otherwise, you emit, and recognise, exactly the names declared by the
    user, and no others

(c) all such input can be read by a simple LR(1) scanner based on an FSM;
    the apparatus of 'lex' and 'yacc' (even supposing one would ever want
    to use them for anything) is not needed.

(d) you read according to the lexis of an 'identifier'; you're required
    to skip leading white space, and of course you need a one-char lookahead
    or putback.

(e) the error behaviour in Ada is to raise DATA_ERROR if there isn't an
    identifier to be read, or if the identifier read isn't a value of the
    type.