Refactor dice amount parser to reusable parsers.

This follows the example in the Combine documentation to use impl
trait return types so we can isolate the parsers into their own
functions.

src/parser.rs

@@ -1,4 +1,6 @@
+use combine::error::ParseError;
 use combine::parser::char::{digit, letter, spaces};
+use combine::stream::Stream;
 use combine::{many, many1, one_of, Parser};
 use thiserror::Error;
 
@@ -73,20 +75,14 @@ pub struct Amount {
     pub element: Element,
 }
 
-/// Converts "+" or" -" into an Operator. No sign at all is an implied
+/// Parser that attempt to convert the text at the start of the dice
-/// Plus. Part of the parser.
+/// parsing into an Amount instance.
-fn map_operator(sign: char) -> Operator {
+fn first_amount_parser<Input>() -> impl Parser<Input, Output = ParseResult<Amount>>
-    match sign {
+where
-        '+' => Operator::Plus,
+    Input: Stream<Token = char>,
-        '-' => Operator::Minus,
+    Input::Error: ParseError<Input::Token, Input::Range, Input::Position>,
-        _ => Operator::Plus,
+{
-    }
+    let map_first_amount = |value: String| {
-}
-
-/// Attempt to convert the text at the start of the expression,
-/// extracted by the Combine parser, into an Amount instance. Part of
-/// the parser.
-fn map_first_amount(value: String) -> ParseResult<Amount> {
         if value.chars().all(char::is_numeric) {
             let num = value.parse::<i32>()?;
             Ok(Amount {
@@ -99,27 +95,51 @@ fn map_first_amount(value: String) -> ParseResult<Amount> {
                 element: Element::Variable(value),
             })
         }
+    };
+
+    many1(letter())
+        .or(many1(digit()))
+        .skip(spaces().silent()) //Consume any space after first amount
+        .map(map_first_amount)
 }
 
-/// Attempt to convert some text in the middle or end of the string,
+/// Attempt to convert some text in the middle or end of the dice roll
-/// extracted by the Combine parser, into an Element. Part of the
+/// string into an Amount.
-/// parser.
+fn amount_parser<Input>() -> impl Parser<Input, Output = ParseResult<Amount>>
-fn map_element(value: String) -> ParseResult<Element> {
+where
+    Input: Stream<Token = char>,
+    Input::Error: ParseError<Input::Token, Input::Range, Input::Position>,
+{
+    let plus_or_minus = one_of("+-".chars());
+    let parse_operator = plus_or_minus.map(|sign: char| match sign {
+        '+' => Operator::Plus,
+        '-' => Operator::Minus,
+        _ => Operator::Plus,
+    });
+
+    // Element must either be a proper i32, or a variable name.
+    let map_element = |value: String| -> ParseResult<Element> {
         if value.chars().all(char::is_numeric) {
             let num = value.parse::<i32>()?;
             Ok(Element::Number(num))
         } else {
             Ok(Element::Variable(value))
         }
-}
+    };
 
-/// Collect an Operator and Element into an Amount, but only if the
+    let parse_element = many1(letter()).or(many1(digit())).map(map_element);
-/// Element was successfully parsed. Part of the parser.
+
-fn map_amount((operator, element_result): (Operator, ParseResult<Element>)) -> ParseResult<Amount> {
+    let element_parser = parse_operator
-    match element_result {
+        .skip(spaces().silent())
+        .and(parse_element)
+        .skip(spaces().silent());
+
+    let convert_to_amount = |(operator, element_result)| match element_result {
         Ok(element) => Ok(Amount { operator, element }),
         Err(e) => Err(e),
-    }
+    };
+
+    element_parser.map(convert_to_amount)
 }
 
 /// Parse an expression of numbers and/or variables into elements
@@ -135,29 +155,8 @@ fn map_amount((operator, element_result): (Operator, ParseResult<Element>)) -> P
 pub fn parse_amounts(input: &str) -> ParseResult<Vec<Amount>> {
     let input = input.trim();
 
-    // Single sub-parser for the first amount expression, because it's
+    let remaining_amounts = many(amount_parser()).map(|amounts: Vec<ParseResult<Amount>>| amounts);
-    // easier. Wraps into ParseResult.
+    let mut parser = first_amount_parser().and(remaining_amounts);
-    let first_amount = many1(letter())
-        .or(many1(digit()))
-        .skip(spaces().silent()) //Consume any space after first amount
-        .map(map_first_amount);
-
-    // All of this, down to amount_parser, will convert expressions
-    // after the first into Amounts (wrapped in a ParseResult).
-    let plus_or_minus = one_of("+-".chars());
-    let maybe_sign = plus_or_minus.map(map_operator);
-
-    let variable_or_number = many1(letter()).or(many1(digit())).map(map_element);
-
-    let element_parser = maybe_sign
-        .skip(spaces().silent())
-        .and(variable_or_number)
-        .skip(spaces().silent());
-
-    let amount_parser = element_parser.map(map_amount);
-    let remaining_amounts = many(amount_parser).map(|amounts: Vec<ParseResult<Amount>>| amounts);
-
-    let mut parser = first_amount.and(remaining_amounts);
 
     // Collapses first amount + remaining amounts into a single Vec,
     // while collecting extraneous input.