Add docs about multiple start rules.

Author: ratmice

doc/src/SUMMARY.md

@@ -15,6 +15,7 @@
   - [grmtools parsing idioms](parsing_idioms.md)
   - [Error recovery](errorrecovery.md)
   - [An AST evaluator](ast_example.md)
+  - [Multiple start rules](parsing_startrules.md)
 - [Rust Editions](editions.md)
 - [The individual libraries and tools](libsandtools.md)
   - [lrpar](lrpar.md)

doc/src/parsing_startrules.md

@@ -0,0 +1,148 @@
+# Multiple start rules
+
+lrpar parsers start with a single start rule, this chapter discusses
+the case where you want _multiple_ start rules.
+
+## Simulating multiple start rules
+
+An approach to simulating multiple start rules from a single start rule
+is to synthesize tokens not produced by the lexer as initial start tokens.
+Then produce those as the initial token passed to the parser to select between
+parser entry points.
+
+### Initial project
+First create a new project, add the dependencies and build dependencies.
+
+```console
+cargo new multistart
+cd multistart
+cargo add lrlex lrpar --build
+cargo add lrlex lrpar cfgrammar
+```
+
+### Parser
+Here is our grammar `src/multistart.y` the tokens `START_A` and `START_B` are
+our synthetic tokens. While `ARule` and `BRule` are our different start rules
+we want to select between.
+
+```
+%grmtools{yacckind: Grmtools}
+%token START_A START_B
+%start meta_start
+%%
+meta_start -> Result<String, DefaultLexeme> :
+  START_A ARule { $2 }
+| START_B BRule { $2 }
+;
+
+ARule -> Result<String, DefaultLexeme>
+: 'A' { Ok($lexer.span_str($1?.span()).to_string()) }
+| BRule { $1 }
+;
+
+BRule -> Result<String, DefaultLexeme>
+: 'B' { Ok($lexer.span_str($1?.span()).to_string()) }
+;
+%%
+use lrlex::DefaultLexeme;
+```
+
+### Lexer
+
+In our lexer we'll need to add a directive to advise that the checker that we expect the missing tokens.
+
+```
+%expect-missing "START_A" "START_B"
+%%
+A "A"
+B "B"
+[\ \n\t] ;
+```
+
+Following this we'll need to prepend one of the start tokens to the lexical analysis phase.
+To indicate which parse rule we expect to use as the start rule. For that we'll need a `token_map`.
+
+### Build.rs
+
+```rust
+use lrlex::{CTLexerBuilder, CTTokenMapBuilder, DefaultLexerTypes};
+use lrpar::CTParserBuilder;
+
+fn main() {
+    // In addition to the usual parser, and lexer
+    // We'll need a token map for our synthetic tokens.
+    let ctp = CTParserBuilder::<DefaultLexerTypes>::new()
+        .grammar_in_src_dir("multistart.y")
+        .unwrap()
+        .build()
+        .unwrap();
+    CTLexerBuilder::new()
+        .rule_ids_map(ctp.token_map())
+        .lexer_in_src_dir("multistart.l")
+        .unwrap()
+        .build()
+        .unwrap();
+    CTTokenMapBuilder::new("token_map", ctp.token_map())
+        .allow_dead_code(true)
+        .build()
+        .unwrap();
+}
+```
+
+### Wrapping the lexer
+
+Finally we'll need to:
+
+1. Run our input text through the parser.
+2. Prepend the selected start token.
+3. Run the parser with the combined tokens.
+
+```rust
+use cfgrammar::NewlineCache;
+use lrlex::LRLexError;
+use lrlex::{DefaultLexeme, DefaultLexerTypes, LRNonStreamingLexer, lrlex_mod};
+use lrpar::Lexer as _;
+use lrpar::{Lexeme, lrpar_mod};
+
+lrlex_mod!("multistart.l");
+lrpar_mod!("multistart.y");
+
+lrlex_mod!("token_map");
+use token_map::{T_START_A, T_START_B};
+
+fn lrlex_wrapper(s: &str, start_token: u32) -> LRNonStreamingLexer<'_, '_, DefaultLexerTypes> {
+    let nl_cache = NewlineCache::new();
+    let lexerdef = multistart_l::lexerdef();
+    let mut tokens: Vec<Result<DefaultLexeme, LRLexError>> =
+        vec![Ok(DefaultLexeme::new(start_token, 0, 0))];
+    let lexer = lexerdef.lexer(s);
+    tokens.extend(lexer.iter());
+    LRNonStreamingLexer::new(s, tokens, nl_cache)
+}
+
+fn main() {
+    assert!(
+        multistart_y::parse(&lrlex_wrapper("A", T_START_A))
+            .0
+            .is_some()
+    );
+    assert!(
+        multistart_y::parse(&lrlex_wrapper("B", T_START_B))
+            .0
+            .is_some()
+    );
+    // This should parse okay, since `START_A` takes either "A", or "B" as input.
+    assert!(
+        multistart_y::parse(&lrlex_wrapper("B", T_START_A))
+            .0
+            .is_some()
+    );
+
+    // However the following should error since `START_B` only takes "B" as input.
+    assert!(
+        !multistart_y::parse(&lrlex_wrapper("A", T_START_B))
+            .1
+            .is_empty()
+    );
+}
+```