choice function

src/choice.rs

@@ -1,6 +1,6 @@
-use crate::parser::Parser;
+use crate::parser::{ParseResult, Parser};
 
-pub fn choice<P1, P2, I, O, E>(parser1: P1, parser2: P2) -> impl Parser<I, O, E>
+pub fn choice2<P1, P2, I, O, E>(parser1: P1, parser2: P2) -> impl Parser<I, O, E>
 where
     P1: Parser<I, O, E>,
     P2: Parser<I, O, E>,
@@ -12,6 +12,43 @@ where
     }
 }
 
+pub fn choice<C, I, O, E>(choices: C) -> impl Parser<I, O, E>
+where
+    C: Choice<I, O, E>,
+    I: Clone,
+{
+    move |input| choices.parse(input)
+}
+
+pub trait Choice<I: Clone, O, E> {
+    fn parse(&self, input: I) -> ParseResult<I, O, E>;
+}
+
+impl<I, O, E, P1, P2> Choice<I, O, E> for (P1, P2)
+where
+    P1: Parser<I, O, E>,
+    P2: Parser<I, O, E>,
+    I: Clone,
+{
+    fn parse(&self, input: I) -> ParseResult<I, O, E> {
+        let parser1 = &self.0;
+        let parser2 = &self.1;
+
+        //TODO need a more principled way to return an error when no choices work
+        let mut err = None;
+
+        for parser in [parser1 as &dyn Parser<I, O, E>, parser2].iter() {
+            match parser.parse(input.clone()) {
+                Ok(result) => return Ok(result),
+                Err(e) => {
+                    err = Some(e);
+                }
+            }
+        }
+        Err(err.unwrap())
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -20,7 +57,7 @@ mod tests {
 
     #[test]
     fn test_choice() {
-        let p = choice(literal("gnostika").to(1), one_or_more(literal(" ")).to(2));
+        let p = choice2(literal("gnostika").to(1), one_or_more(literal(" ")).to(2));
         assert_eq!(p.parse("gnostika twentynine"), Ok((1, " twentynine")));
     }
 }

src/lib.rs

@@ -45,7 +45,7 @@ mod tests {
         let json_true = literal("true").to(JsonValue::Bool(true));
         let json_false = literal("false").to(JsonValue::Bool(false));
 
-        let json_value = choice(json_null, choice(json_true, json_false));
+        let json_value = choice((json_null, choice((json_true, json_false))));
 
         assert_eq!(json_value.parse("true"), Ok((JsonValue::Bool(true), "")));
     }