[MLton] confusing error message from MLton

[Stephen Weeks]

> But, I don't know why the grammer isn't parsing the code with = as an
> infix.  It seems like the following would be a valid parse:
...
> That is, parse the equals as infix, applied to fiba applied to 4 arguments
> and to the case expression.

Yes, your example is valid.  It fails without the parens because case
expressions need to be parenthesized in an infix expression.  Have a
look at the grammar on page 63 of the Definition.

> Presumably, the attempt to insert a VAL arises from the use of =.

Yeah, if you replace the "=" by "foo", then the parser quite
reasonably inserts a left paren just before the case expression.  I
don't understand why "=" is treated differently, since it is a valid
longid in an infix expression.  I don't have a good understanding of
how the error correction works in ML-Yacc works though.

Henry, if you want to take a closer look, in the ML-Yacc User's
Manual, section 1.3 talks about error recovery, and there is also a
reference to a TOPLAS paper on error recovery at the end of the
manual.  In ml.grm, the error recovery is specified by the %change
directive, which says:

%change -> VAL | -> THEN | -> ELSE | -> LPAREN | -> SEMICOLON | 
        DARROW -> EQUALOP | EQUALOP -> DARROW | AND -> ANDALSO | COLON -> OF |
        SEMICOLON -> COMMA | COMMA -> SEMICOLON |
        -> IN LONGID END | -> ELSE LONGID

I guess that the first entry, "-> VAL" is responsible for the message.
I suspect that this entire directive is copied unchanged from SML/NJ's
parser.  I've never had enough trouble fixing parse errors to make it
worth investigating further.