RFC: Unicode primes and super/subscript characters in GHC

[Mateusz Kowalczyk]

On 06/14/2014 04:48 PM, Mikhail Vorozhtsov wrote:
> Hello lists,
> 
> As some of you may know, GHC's support for Unicode characters in lexemes 
> is rather crude and hence prone to inconsistencies in their handling 
> versus the ASCII counterparts. For example, APOSTROPHE is treated 
> differently from PRIME:
> 
> λ> data a +' b = Plus a b
> <interactive>:3:9:
>      Unexpected type ‘b’
>      In the data declaration for ‘+’
>      A data declaration should have form
>        data + a b c = ...
> λ> data a +′ b = Plus a b
> 
> λ> let a' = 1
> λ> let a′ = 1
> <interactive>:10:8: parse error on input ‘=’
> 
> Also some rather bizarre looking things are accepted:
> 
> λ> let ᵤxᵤy = 1
> 
> In the spirit of improving things little by little I would like to propose:
> 
> 1. Handle single/double/triple/quadruple Unicode PRIMEs the same way as 
> APOSTROPHE, meaning the following alterations to the lexer:
> 
> primes -> U+2032 | U+2033 | U+2034 | U+2057
> symbol -> ascSymbol | uniSymbol (EXCEPT special | _ | " | ' | primes)
> graphic -> small | large | symbol | digit | special | " | ' | primes
> varid -> (small { small | large | digit | ' | primes }) (EXCEPT reservedid)
> conid -> large { small | large | digit | ' | primes }
> 
> 2. Introduce a new lexer nonterminal "subsup" that would include the 
> Unicode sub/superscript[1] versions of numbers, "-", "+", "=", "(", ")", 
> Latin and Greek letters. And allow these characters to be used in names 
> and operators:
> 
> symbol -> ascSymbol | uniSymbol (EXCEPT special | _ | " | ' | primes | 
> subsup )
> digit -> ascDigit | uniDigit (EXCEPT subsup)
> small -> ascSmall | uniSmall (EXCEPT subsup) | _
> large -> ascLarge | uniLarge (EXCEPT subsup)
> graphic -> small | large | symbol | digit | special | " | ' | primes | 
> subsup
> varid -> (small { small | large | digit | ' | primes | subsup }) (EXCEPT 
> reservedid)
> conid -> large { small | large | digit | ' | primes | subsup }
> varsym -> (symbol (EXCEPT :) {symbol | subsup}) (EXCEPT reservedop | dashes)
> consym -> (: {symbol | subsup}) (EXCEPT reservedop)
> 
> If this proposal is received favorably, I'll write a patch for GHC based 
> on my previous stab at the problem[2].
> 
> P.S. I'm CC-ing Cafe for extra attention, but please keep the discussion 
> to the GHC users list.
> 
> [1] https://en.wikipedia.org/wiki/Unicode_subscripts_and_superscripts
> [2] https://ghc.haskell.org/trac/ghc/ticket/5108
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> 

While personally I like the proposal (wanted prime and sub/sup scripts
way too many times), I worry what this means for compatibility reasons:
suddenly we'll have code that fails to build on 7.8 and before because
someone using 7.9/7.10+ used ′ somewhere. Even using CPP based on
version of the compiler used is not too great in this scenario because
it doesn't bring significant practical advantage to justify the CPP
clutter in code. If the choice is either extra lines due to CPP or using
‘'’ instead of ‘′’, I know which I'll go for.

I also worry (although not based on anything particular you said)
whether this will not change meaning of any existing programs. Does it
only allow new programs?

Will it be enabled by a pragma?

I simply worry about how practical it will be to use for actual programs
and libraries that will go out on Hackage and wider world, even if it is
accepted.

-- 
Mateusz K.