Discover the magic of nom::combinator::value

2022/src/day05.rs

@@ -6,6 +6,7 @@ use nom::bytes::complete::take_until;
 use nom::character::complete::newline;
 use nom::combinator::map;
 use nom::combinator::opt;
+use nom::combinator::value;
 use nom::multi::fold_many1;
 use nom::multi::many1;
 use nom::sequence::delimited;
@@ -31,7 +32,7 @@ fn parse_row<'a>(input: &'a [u8], stacks: &mut OwnedStacks) -> IResult<&'a [u8],
                     Some(v[0])
                 }),
                 // Or an empty stack into a None
-                map(tag("   "), |_| None),
+                value(None, tag("   ")),
             )),
             opt(tag(" ")),
         )),

2022/src/day10.rs

@@ -4,6 +4,7 @@ use nom::bytes::complete::tag;
 use nom::character::complete::newline;
 use nom::combinator::iterator;
 use nom::combinator::map;
+use nom::combinator::value;
 use nom::sequence::preceded;
 use nom::sequence::terminated;
 use nom::IResult;
@@ -36,7 +37,7 @@ impl Instruction {
 fn parse_instruction(input: &[u8]) -> IResult<&[u8], Instruction> {
     terminated(
         alt((
-            map(tag("noop"), |_| Instruction::Noop),
+            value(Instruction::Noop, tag("noop")),
             map(preceded(tag("addx "), nom::character::complete::i32), |v| {
                 Instruction::AddX(v)
             }),

2022/src/day11.rs

@@ -6,6 +6,7 @@ use nom::character::complete::digit1;
 use nom::character::complete::newline;
 use nom::combinator::map;
 use nom::combinator::map_res;
+use nom::combinator::value;
 use nom::multi::separated_list0;
 use nom::multi::separated_list1;
 use nom::sequence::delimited;
@@ -81,7 +82,7 @@ fn parse_operation(input: &[u8]) -> IResult<&[u8], Operation> {
                     _ => Err(anyhow::anyhow!("Invalid operation {op:?}")),
                 },
             ),
-            map(tag("* old"), |_| Operation::Square),
+            value(Operation::Square, tag("* old")),
         )),
     )(input)
 }