[Discussion] Character class for apostrophe

[gracjan]

Apostrophe is seriously overloaded character in Haskell. Here it goes:

• 'x', 'X' - a char constant
• 'xf' - Template Haskell Name for xf'
• ''XF' - Template Haskell Type for XF'
• '\1231' - a char constant
• '\SOH' - a char constant
• '' - illegal, somewhat char
• '1x' - illegal
• '''XF - illegal

So apostrophe is a:

• symbol constituent (xf')
• string quote ('x')
• expression prefix ('xf ''XF)
• illegal

Most of the above can be fixed by syntax-propertize. There are two questions though:

• what should be the default syntax class for apostrophe?
• what should be the syntax class for apostrophe in illegal cases?
• should apostrophe that is part of identifiers have syntax class _ or w?

[ivan-m]

Don't forget also that x'f is also a valid variable/function name (so ' isn't just a symbol constituent at the end).

[gracjan]

Proposition is:

• default for apostrophe is punctuation
• illegal apostrophe is punctuation
• apostrophe in identifiers is same class as underscore, that is 'w'

[gracjan]

Some more testing:

Prelude> (,) 'x''y'
('x','y')
Prelude> (,) 12'y'
(12,'y')
Prelude> (,) 0x12'y'
(18,'y')
Prelude> (,) x12'y'
<interactive>:6:5: Not in scope: ‘x12'y'’
Prelude> (,) 'x''y''
<interactive>:7:12:
    lexical error in string/character literal at end of input
Prelude> let y = 45
Prelude> (,) 'x'y
('x',45)
Prelude> (,) 'x''y
('x',y_1627393257)
Prelude> (,) "x"'y'
("x",'y')
Prelude> (,) 'y'"x"
('y',"x")
Prelude> (,) '"'"'"
('"',"'")
Prelude> (,) 12e-34'r'
(1.2e-33,'r')
Prelude> (,) 12.34'r'
(12.34,'r')

[gracjan]

And apostrophe for DataKinds and TemplateHaskell is prefix operator class.

[gracjan]

This is more or less fixed in the new syntax-propertize0-function.