SML Error: syntax error: inserting DOT

Question

When I run this code in the REPL, it throws Error: syntax error: inserting DOT. I would like to know what that error message means.

I have since fixed the code and still want to know the meaning of that message for future reference.

part_dir is function that returns a custom datatype direction with possible patterns Left and Right.

fun same (fs)=
    case fs of
         (f1::f2::fs') => case (part_dir(f1),part_dir(f2)) of
                               (dir1=dir2) => same (f2::fs')
                             | _           => false
        | _            => true

sepp2k · Accepted Answer · 2013-01-26 19:38:40Z

4

"syntax error: inserting DOT" means that there's an unexpected token at the position it's complaining about and that a dot would be legal at that position. However that does not mean that a dot would be the only thing allowed in that position or that replacing the token with a dot would fix your error.

For the most part the error message is useless beyond telling you that there's a syntax error at that position. You should probably just ignore the "inserting DOT" part as it generally doesn't lead you in the right direction. Just look at the line and column it's complaining about and try to find what the syntax error is there.

In your case the problem is that dir1=dir2 is not a legal pattern. A legal pattern would be the constructor of a datatype with patterns for each of its argument, a variable name, _ or a constant or a tuple of patterns. There is no pattern to say "a tuple that contains two elements that are equal". For that you need the pattern (dir1, dir2) and then the condition dir1=dir2 in an if.

In your case you don't even need an if though. You can just write:

(dir1, dir2) => dir1 = dir2 andalso same (f2 :: fs)

answered Jan 26, 2013 at 19:38

sepp2k

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1 Comment

gotube Over a year ago

Thanks! I figured the message itself probably wouldn't be too helpful, but now I know to treat it as a general syntax error.

Jesper.Reenberg · Accepted Answer · 2013-01-28 01:50:22Z

Also note that your two case expressions doesn't work as you expect them to. Your last pattern match of the outer case, is actually part of the inner case. In other words your code is actually has this meaning

fun same (fs)=
    case fs of
      (f1::f2::fs') => case (part_dir(f1),part_dir(f2)) of
                         (dir1=dir2) => same (f2::fs')
                       | _           => false
                       | _           => true

When fixing the error as pointed out by sepp2k, and your code compiles, you will notice that the interpretor spits out a warning and an error. First the error, that the last match of the inner case is redundant, and then the warning that the out case has non-exhaustive matches. That is, it doesn't have cases for all possible inputs.

Thus it is always a good idea to put parentheses around case expressions, as the same case will happen if your function same had other matches below the case. The reason is that the pipe is common syntax where multiple patterns are matched.

With the added parenthesis your function will get the intended meaning

fun same (fs)=
    case fs of
      (f1::f2::fs') => (case (part_dir(f1),part_dir(f2)) of
                         (dir1=dir2) => same (f2::fs')
                       | _           => false)
    | _             => true

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