Re: state in parsers

Hi,

the problem in haskell is that simply specifying indent is not enough, 
for example:

let a = 20
      b = 30

and
let  a = 20
      b = 30

are not the same thing.  The first translates to

let {a = 20
      ;b = 30 }

and the second to (the illegal)

let {a = 20}
      b = 30

And for example the following

let a = 20
      b = 30
      in a + b

translates to

let {a = 20
      ;b = 30
      }in a + b

not to

let {a = 30
      ;b = 30
      ;in a + b}

Which is illegal.  Where to insert ';' depends on the parser really, 
it's hard to do at the lex phase.

Regards,
Kristof Bastiaensen



On 03-11-15 13:22, Eric Ludlam wrote:
> On 11/02/2015 07:13 AM, Kristof Bastiaensen wrote:
>> Hi,
>>
>> I'd like to add support for haskell to semantic.  Haskell syntax is
>> sensitive
>> to indentation, and I'd like to know if it is possible to use bovine or
>> wisent for
>> it.
>
> Yes.
>
>> The layout rule is explained here:
>> https://www.haskell.org/onlinereport/haskell2010/haskellch2.html#x7-210002.7 
>>
>>
>> What it does, is after certain keywords, whenever a '{' character is
>> ommited, it
>> enters a layout context.  In the layout context it will remember the
>> indentation
>> of the next token.  When a line has the same indentation as this token,
>> a ';' token is
>> inserted before the expression.  Whenever the indentation is less than
>> this, a '}' token
>> is inserted, and the layout context is ended.  The layout context is
>> also ended when a parser error would occur.
>
> The python grammar in semantic/wisent/python.wy and python.el is an ok 
> example.  The premise is that when you develop the lexer, you teach it 
> to convert your indentation into lexical tokens that you then use in 
> your grammar.
>
> In the python.el support file, you will notice it uses 
> (current-indentation) and uses that to derive INDENT and DEDENT 
> tokens.  The grammar uses those to create INDENT and DEDENT blocks so 
> it can recurse into code bodies.  You could just as easily skip them 
> too if you prefer.
>
>> So I would set a state variable to all the layout context indentations,
>> and then change the behaviour of the lexer or parser based on this
>> state.  Is that possible?  Would it interfere with incremental parsing?
>
> Definitely start with the lexer, and there is good documentation on 
> creating a lexer in the semantic language developers guide. Use the 
> lexer testing function to make sure it outputs what you expect, and 
> then you can continue on with the grammar.
>
> Eric


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