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10   Hints

10.1   Multiple start symbols

To have multiple start symbols, define a dummy token for each start symbol. Then define a start symbol which derives the multiple start symbols with dummy tokens placed in front of them. When you start the parser you must place a dummy token on the front of the lexer stream to select a start symbol from which to begin parsing.

Assuming that you have followed the naming conventions used before, create the lexer using the makeLexer function in the {n}Parser structure. Then, place the dummy token on the front of the lexer:
val dummyLexer =
        ({n}LrVals.Tokens.{dummy token name}
                 ({dummy lineno},{dummy lineno}),
You have to pass a Tokens structure to the lexer. This Tokens structure contains functions which construct tokens from values and line numbers. So to create your dummy token just apply the appropriate token constructor function from this Tokens structure to a value (if there is one) and the line numbers. This is exactly what you do in the lexer to construct tokens.

Then you must place the dummy token on the front of your lex stream. The structure {n}Parser contains a structure Stream which implements lazy streams. So you just cons the dummy token on to stream returned by makeLexer.

10.2   Functorizing things further

You may wish to functorize things even further. Two possibilities are turning the lexer and parser structures into closed functors, that is, functors which do not refer to types or values defined outside their body or outside their parameter structures (except for pervasive types and values), and creating a functor which encapsulates the code necessary to invoke the parser.

Use the %header declarations in ML-Lex and ML-Yacc to create closed functors. See section 2.4 of this manual and section 4 of the manual for ML-Lex for complete descriptions of these declarations. If you do this, you should also parameterize these structures by the types of line numbers. The type will be an abstract type, so you will also need to define all the valid operations on the type. The signature INTERFACE, defined below, shows one possible signature for a structure defining the line number type and associated operations.

If you wish to encapsulate the code necessary to invoke the parser, your functor generally will have form:
functor Encapsulate(
     structure Parser : PARSER
     structure Interface : INTERFACE
         sharing type Parser.arg = Interface.arg
         sharing type Parser.pos = Interface.pos
         sharing type Parser.result = ...
     structure Tokens : {parser name}_TOKENS
         sharing type Tokens.token = Parser.Token.token
         sharing type Tokens.svalue = Parser.svalue) =
The signature INTERFACE, defined below, is a possible signature for a structure defining the types of line numbers and arguments (types pos and arg, respectively) along with operations for them. You need this structure because these types will be abstract types inside the body of your functor.
signature INTERFACE = 
   type pos
   val line : pos ref
   val reset : unit -> unit
   val next : unit -> unit
   val error : string * pos * pos -> unit

   type arg
   val nothing : arg
The directory example/fol contains a sample parser in which the code for tying together the lexer and parser has been encapsulated in a functor.

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