Question

I’m writing a Ruby gem using the {key: 'value'} syntax for hashes throughout my code. My tests all pass in 1.9.x, but I (understandably) get syntax error, unexpected ':', expecting ')' in 1.8.7.

Is there a best practice for supporting the 1.8.x? Do I need to rewrite the code using our old friend =>, or is there a better strategy?

Answer 1

I think you’re out of luck, if you want to support 1.8 then you have to use =>. As usual, I will mention that you must use => in certain cases in 1.9:

1. If the key is not a symbol. Remember that any object (symbols, strings, classes, floats, …) can be a key in a Ruby Hash.
2. If you need a symbol that you’d quote: :'this.that'.
3. If you use MongoDB for pretty much anything you’ll be using things like :$set => hash but $set: hash is a syntax error.

Back to our regularly scheduled programming.

Why do I say that you’re out of luck? The Hash literal syntaxes (both of them) are hard-wired in the parser and I don’t think you’re going to have much luck patching the parser from your gem. Ruby 1.8.7’s parse.y has this to say:

assoc    : arg_value tASSOC arg_value
             {
                 $$ = list_append(NEW_LIST($1), $3);
             }
         ;

and tASSOC is => so hash literals are hard-wired to use =>. 1.9.3’s says this:

assoc    : arg_value tASSOC arg_value
             {
             /*%%%*/
                 $$ = list_append(NEW_LIST($1), $3);
             /*%
                 $$ = dispatch2(assoc_new, $1, $3);
             %*/
             }
         | tLABEL arg_value
             {
             /*%%%*/
                 $$ = list_append(NEW_LIST(NEW_LIT(ID2SYM($1))), $2);
             /*%
                 $$ = dispatch2(assoc_new, $1, $2);
             %*/
             }
         ;

We have the fat-arrow syntax again (arg_value tASSOC arg_value) and the JavaScript style (tLABEL arg_value); AFAIK, tLABEL is also the source of the restrictions on what sorts of symbols (no :$set, no :'this.that', …) can be used with the JavaScript-style syntax. The current trunk parse.y matches 1.9.3 for Hash literals.

So the Hash literal syntax is hard-wired into the parser and you’re stuck with fat arrows if you want to support 1.8.