From aac3549a72663c523a456b2f5d7c3b77f509cdd6 Mon Sep 17 00:00:00 2001
From: Greg Brown <gmb60@cam.ac.uk>
Date: Thu, 14 Jan 2021 11:42:55 +0000
Subject: Add labelled expressions.

Restructure project (again).

Convert `Cat` and `Alt` from binary to n+2-ary.
---
 chewed/src/parse.rs | 27 ++++++++++++---------------
 1 file changed, 12 insertions(+), 15 deletions(-)

(limited to 'chewed/src')

diff --git a/chewed/src/parse.rs b/chewed/src/parse.rs
index 65e9272..2d01757 100644
--- a/chewed/src/parse.rs
+++ b/chewed/src/parse.rs
@@ -1,3 +1,5 @@
+use std::fmt::Display;
+
 use super::{
     error::{ParseError, TakeError},
     position::LineCol,
@@ -102,7 +104,7 @@ impl<I: ?Sized + Iterator<Item = char>> Parser for IterWrapper<I> {
     }
 }
 
-pub trait Parse: Sized {
+pub trait Parse: Display + Sized {
     fn take<P: Parser + ?Sized>(input: &mut P) -> Result<Self, TakeError>;
 
     fn parse<P: Parser>(mut input: P) -> Result<Self, ParseError> {
@@ -124,23 +126,18 @@ pub trait Parse: Sized {
     }
 }
 
-impl Parse for () {
-    fn take<P: Parser + ?Sized>(_: &mut P) -> Result<Self, TakeError> {
-        Ok(())
-    }
-}
+#[derive(Clone, Copy, Debug, Default, Eq, Hash, PartialEq)]
+pub struct Epsilon;
 
-impl<A: Parse, B: Parse> Parse for (A, B) {
-    fn take<P: Parser + ?Sized>(input: &mut P) -> Result<Self, TakeError> {
-        let a = input.take()?;
-        let b = input.take()?;
-        Ok((a, b))
+impl Display for Epsilon {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "")
     }
+}
 
-    fn parse<P: Parser>(mut input: P) -> Result<Self, ParseError> {
-        let a = A::take(&mut input)?;
-        let b = B::parse(input)?;
-        Ok((a, b))
+impl Parse for Epsilon {
+    fn take<P: Parser + ?Sized>(_: &mut P) -> Result<Self, TakeError> {
+        Ok(Epsilon)
     }
 }
 
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