[racket] Racketrivia: Using ' as an identifier suffix

On Jan 8, 2015, at 3:15 PM, Greg Hendershott <greghendershott at gmail.com> wrote:

> So Haskell conventionally uses ' as a suffix, prime. From what I've
> seen, Scheme and Racket tend to use * instead.
> 
> At some point I "learned" that you cannot use ' as a suffix in Racket.
> 
> Today I tried again, and was surprised to see that it works... somewhat.
> 
> $ racket
> Welcome to Racket v6.1.1.6.
> -> (define x' 42)

This definition introduces x as '42. Works like a charm 


> -> x'

This reference derefs x, which is 42. And it introduces a ' into the io stream. 


> 42
> -> (+ x' 10)

Here the quote is applied to an S-expression and you get exactly the expected result. 


> '(+ x '10)
> -> (+ 10 x')

This is + 10 x [as defined above] and the ' token. There is one place where it can't appear: to the left of a )


> ; readline-input:4:8: read: unexpected `)'
> -> (+ 10 x' )

Same here. 


;; --- 

In principle, ' would be good to use as part of a name but I prefer to use it for quoted S-expressions if that's the choice I have. Then again, I'd be willing to give up ' in favor of ` if this didn't have performance problems. 

-- Matthias













> ; readline-input:5:8: read: unexpected `)'
> 
> 
> 0. It turns out x' _is_ a valid identifier, and it self-evaluates just
> fine. Interesting.
> 
> 1. I don't understand why (+ x' 10) evaluates not to 52, and not even
> an error, but... '(+ x '10).  WAT.
> 
> 2. Less surprising to me is that (+ 10 x') and even (+ 10 x' ) are
> errors. But actually, I wonder why the reader (or lexer?) couldn't
> handle ' followed by a character that can't be part of an identifier?
> 
> 
> p.s. I'm not proposing this would be a great suffix style to use.
> Quick, distinguish x' from 'x ! And don't type one when you mean the
> other! I get that. Even so, I'm curious.
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