Re: minor LPEG doubt

[joy mondal <joykrishnamondal@...>]

Hi Sean Conner,

Thanks for the quick reply !

You have answered both my questions

( I need to try and write the first example since I am still unsure about the LPEG code for capturing indentation - left curly and right curly are simple enough but I am thrown off regarding indentation ).

For the first issue, yes I need to create a hierarchical tree instead of a flat output, normally examples of lexer output online show a stream of tokens, but LPEG creates an AST directly.

The second one is my fault, I was unaware that #table was the syntax for getting table length.

Cheers !

On Sat, Sep 15, 2018 at 3:47 AM Sean Conner <sean@conman.org> wrote:

It was thus said that the Great joy mondal once stated:
> Hi !
>
> Lets say you have a language that looks as follows :
>
> hello =
>   (a,b) ->
>     print 'hello world'
>     1 + 2
>
> ( It looks a lot like moonscript if that helps )
>
> hello = \n  (a,b) ->\n    print 'hello world'\n    (1 + 2)\n
>
>
> Would the token stream out of your LPEG grammer look ideally like this ?

  Are you asking a particular person or just anyone in general? 

  LPeg doesn't automatically parse text---it's a library of code to generate
code that will parse text.

> [var hello]
>
>
> [assign]
>
> [indent 2]
>
> [function start a b]
>
> [indent 4]
>
> [call print 'hello world']
>
> [indent 4]
>
> [call add 1 2]
>
> [indent 2]

  Yes, the output of an LPeg parser *could* output that, but someone would
have to write the code to do so.  The LPeg I might write might return the
following Lua table:

        {
          nodetype = 'assignment',
          to = 'hello',
          value =
          {
            nodetype = 'function',
            input = { 'a' , 'b' },
            code =
            {
              {
                nodetype = 'call',
                id = 'print',
                parameters = { 'hello world' }
              },
              {
                nodetype = '_expression_',
                code =
                {
                  nodetype = '+',
                  parameters =
                  {
                    {
                      exprtype = 'immediate',
                      type = 'number',
                      value = 1
                    },
                    {
                      exprtype = 'immediate',
                      type = 'number',
                      value = 2
                    }
                  }
                }
              }
            }
          }
        }

  It all depends upon the LPeg code.

> One final issue, when using -- lpeg.Ct -- How would I iterate over the
> table since I cannot call .length on it ?

  The table returned from lpeg.Ct() is a Lua table and can be iterated over
like any other Lua table.

        parse = lpeg.Ct(lpeg.C("a")^0)

        x = parse:match "aaaaaaaa"
        print(#x,x)
        for i = 1 , #x do print(x[i]) end

  If you mean "How do I obtain the length of the table returned by lpeg.Ct()
during parsing?" then something like this would work:

        parse = lpeg.Cf(lpeg.Ct"" * lpeg.C"a"^0,function(t,a)
          if #t < 5 then
            table.insert(t,a)
          end
          return t
        end)

        x = parse:match "aaaaaaaa"
        print(#x,x)
        for i = 1 , #x do print(x[i]) end

  There are other ways to accomplish the same thing, but this is better
served by this:

        parse = lpeg.Ct(lpeg.C"a"^-5) * lpeg.P"a"^0

        x = parse:match "aaaaaaaa"
        print(#x,x)
        for i = 1 , #x do print(x[i]) end

where you look for the maximum number of items you want in the table.

  It might help us to know what exactly you are trying to accomplish with
LPeg.

  -spc (And do try the examples and work out how they work)