Ambiguous Optional Parentheses, Part Three

Ambiguous Optional Parentheses, Part Three

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(This is part three of a three-part series on C# language design issues involving elided parentheses in constructor calls. Part two is here.)

Last time I discussed why a particular syntactic sugar would have been rejected by the design team: because it introduces the costs of a nasty semantic analysis ambiguity without any corresponding benefit. Continuing on with our dialogue...

How did you determine that particular ambiguity?

I am pretty familiar with the rules in C# for determining when a dotted name is expected and how the semantic analyzer resolves it. A few minutes double-checking the spec and trying out some sample programs in the compiler confirmed my initial suspicions.

Fair enough. But when considering a new feature how do you in general determine whether it causes any ambiguity? By hand? by formal proof? by machine analysis? what?

All three. Mostly we just look at the spec and noodle on it, as I did for the proposed feature last time.

Let me take you through an example of how we might noodle around the spec looking for ambiguities. Suppose hypothetically we wanted to add a new prefix unary operator to C#, called frob:

(UPDATE: I can now tell you that this was the exact analysis we went through when adding "async" to the language; these are all cases we considered when adding that prefix unary operator.)

x = frob 123 + 456

frob here is like new or ++ - it comes before an expression of some sort.

First thing we'd work out is the desired precedence and associativity and so on, so that we know whether this means frob(123+456) or (frob 123)+456

Then we'd start asking questions like "what if the program already has a type, field, property, event, method, constant, or local called frob?" That would immediately lead to cases like:

frob x = 10;

does that mean "do the frob operation on the result of x = 10, or create a variable of type frob called x and assign 10 to it?" Or consider this case:

G(frob + x);

Does that mean "frob the result of the unary plus operator on x" or "add expression frob to x"?

And so on. To resolve these ambiguities we might introduce heuristics. We've seen these sorts of problems before, after all. When you say var x = 10; that's ambiguous; it could mean "infer the type of x" or it could mean "x is of type var". So we have a heuristic: we first attempt to look up a type named "var", and only if one does not exist do we infer the type of x.

Or, we might change the syntax so that it is not ambiguous. When they designed C# 2.0 they had this problem:

yield(x);

Does that mean "yield x in an iterator" or "call the yield method with argument x?" By changing it to

yield return(x);

it is now unambiguous; it cannot possibly mean "call the yield method".

In the case of optional parens in an object initializer argument list it is straightforward to reason about whether there are ambiguities introduced or not. Why? Because the argument initializer has a left curly brace after the constructor. The number of situations in which it is permissible to introduce something that starts with a left curly brace is very small. Basically just various statement contexts, statement lambdas, array initializers and that's about it. It's easy to reason through all the cases and show that there's no ambiguity. Making sure the IDE stays efficient is somewhat harder (because you have to deal with partially-edited programs that might have braces in strange places) but can be done without too much trouble.

This sort of fiddling around with the spec usually is sufficient. If it is a particularly tricky feature then we pull out heavier-duty tools. For example, when designing LINQ, one of the compiler guys and one of the IDE guys who both have a background in parser theory built themselves a parser generator that could analyze grammars looking for ambiguities, and then fed proposed C# grammars for query comprehensions into it; doing so found many cases where queries were ambiguous.

If a feature is really quite tricky and we want to get a professional opinion on it, we'll call in the super geniuses. For example, when we did advanced type inference on lambdas in C# 3.0 we wrote up our proposals and then sent them over the pond to Microsoft Research in Cambridge where the languages team there was good enough to work up a formal proof that the type inference proposal was theoretically sound.

(This is part three of a three-part series on C# language design issues involving elided parentheses in constructor calls. Part two is here.) 

  • Why does this post sound so familiar? Where have I heard you mention the frob example before?

  • @configurator: As Eric Mentions Part One of this series, this article is based on a StackOverflow question.  This article and his answer to that question both contain the frob example.  Of course, you'll see "frob" all over the place in Eric's blog posts as a function name.

  • i wrote a compiler and changed the C/C++ syntax.

    For the address of a function, you must now put "&" before the function name.

    If a function has no parameters or has defaults, no parenthesis needed.

    What part of "wrote compiler" do you not understand?

  • "That would immediately lead to cases like 'frob x = 123;' does that mean 'do the frob operation on the result of x = 10, or create a variable of type frob called x and assign 10 to it?'"

    I'd hope it would mean neither of those things, or it could get confusing in a hurry.... ;)

  • Another trick for the toolbox: implement your grammar using ANTLR (if possible). Even if you don't plan on using the generated code, they have a great visual tool that gives you a lot of insight into the grammar. It even finds ambiguities automatically.

  • "If a feature is really quite tricky and we want to get a professional opinion on it, we'll call in the super geniuses".  This phrase really struck me!  For *you* to consider somebody to be a super genius they really must be smart!

    Very nice article, as ever it's incredibly interesting to see the things that you deal with in your day to day.

    I wouldn't mind another music article though! ;-)  I just read through "Desafinado" again, it's one of my favourites. :-)

  • Do the super-geniuses wear capes? Is there a special way to signal that you need their help?

    Good series (as always)! Turns out, designing a programming language is hard, who knew?

  • "theoretically sound" or "sound theoretically"? ;)

  • You mean you're not a super genius? O.O

    You had me fooled ;)

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