Switch statement syntax

Monday, October 25, 2010
Here's a few minor design puzzles. C's "switch" statement requires a "break" statement at the end of a switch case. I've long felt that this was a design misfeature, and while it is useful for some clever hacks, it's also a source of much trouble. I decided instead to make each case a first-class block. Here's a sample of what switch statements look like in Tart:

   switch value {
     case 1 {
       // something
     }

     case 2
     case 3 {
       // something
     }

     else {
       // something
     }
   }

Note that cases 2 & 3 go to the same block. Syntactically, the rule is that a case value can be followed by one of two things: Another case value, or the start of a statement block.

One alternative syntax I considered was to use a comma to separate case values: "case 2, 3". However, I discarded this for two reasons: first, because it looks ugly if there are a lot of case values and you need to break the line, and second, because I eventually want to be able to have case values that are tuples.

However, the lack of a delimiter after the case value is a bit troubling. I've discovered at least one bug where I typed a case value and forgot to write the subsequent block, which resulted in the executing falling through to the next block. If a delimiter were required, then the compiler would have caught that.

There's also a problem with the use of the keyword 'else' instead of 'default' - sometimes I like to put the default case first instead of last, and that doesn't read as well with the word 'else'.

Finally, there is the issue of the 'functional-switch'. A lot of functional languages have a switch statement which returns a value (let n = switch value { ... }). I've been wondering if Tart should have something like this. However, it's not clear to me how you would indicate which value to return in a block, especially in an imperative language which supports multiple statements. The choices are (a) return the value of the last statement in the block (if there is one), (b) have a special keyword for returning a value of a block. Both approaches have issues.

Having a functional-switch implies that there should also be a functional-if as well, which is nothing more than the C/Java/Python conditional operator. Tart doesn't have or need such an operator, since the 'cond' macro does the same job without needing special syntactic sugar. However, doing a switch statement as a macro is a little more difficult, since the number of arguments to the macro can grow arbitrarily large.

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