RFC: Unicode primes and super/subscript characters in GHC

[John Meacham]

Don't forget that for every line of haskell code on hackage there are
dozens of lines used internally within organizations where
compatibility beyond their target internal tools may not be a concern.
Deciding on a policy of allowing primes or whatnot within an
organization seems quite plausible and doesn't entail CPP concerns.

    John

On Sun, Jun 15, 2014 at 5:26 PM, Mateusz Kowalczyk
<fuuzetsu at fuuzetsu.co.uk> wrote:
> 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
>> _______________________________________________
>> Glasgow-haskell-users mailing list
>> Glasgow-haskell-users at haskell.org
>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
>>
>
> 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.
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> Glasgow-haskell-users at haskell.org
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-- 
John Meacham - http://notanumber.net/