From 3d9dfaeadf85c31295b2b14ac25bd66ddc774177 Mon Sep 17 00:00:00 2001
From: NanoTech
Date: Tue, 6 Dec 2016 21:22:01 -0600
Subject: rustc-style error messages

---
 Cargo.lock  |  7 +++++++
 Cargo.toml  |  1 +
 src/main.rs | 32 +++++++++++++++++++++++++++++---
 3 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index 50a3b6b..9cc468a 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2,6 +2,7 @@
 name = "dhall"
 version = "0.1.0"
 dependencies = [
+ "bytecount 0.1.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "lalrpop 0.12.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "lalrpop-util 0.12.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "nom 2.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -43,6 +44,11 @@ name = "bitflags"
 version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
+[[package]]
+name = "bytecount"
+version = "0.1.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
 [[package]]
 name = "diff"
 version = "0.1.9"
@@ -246,6 +252,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 "checksum bit-set 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "84527c7b0452f22545cc010e72d366a435561d2b28b978035550b3778c4d428d"
 "checksum bit-vec 0.4.3 (registry+https://github.com/rust-lang/crates.io-index)" = "5b97c2c8e8bbb4251754f559df8af22fb264853c7d009084a576cdf12565089d"
 "checksum bitflags 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "8dead7461c1127cf637931a1e50934eb6eee8bff2f74433ac7909e9afcee04a3"
+"checksum bytecount 0.1.4 (registry+https://github.com/rust-lang/crates.io-index)" = "49e3c21915578e2300b08d3c174a8ac887e0c6421dff86fdc4d741dc29e5d413"
 "checksum diff 0.1.9 (registry+https://github.com/rust-lang/crates.io-index)" = "e48977eec6d3b7707462c2dc2e1363ad91b5dd822cf942537ccdc2085dc87587"
 "checksum docopt 0.6.86 (registry+https://github.com/rust-lang/crates.io-index)" = "4a7ef30445607f6fc8720f0a0a2c7442284b629cf0d049286860fae23e71c4d9"
 "checksum fixedbitset 0.1.5 (registry+https://github.com/rust-lang/crates.io-index)" = "88c3c33fc4c00db33f5174eb98aea809c4c007db0b71351d810a7e094ea3b64d"
diff --git a/Cargo.toml b/Cargo.toml
index 625cda1..8a5ad81 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -8,5 +8,6 @@ build = "build.rs"
 lalrpop = "0.12.4"
 
 [dependencies]
+bytecount = "0.1.4"
 lalrpop-util = "0.12.4"
 nom = "2.0.0"
diff --git a/src/main.rs b/src/main.rs
index e60a893..0abcea8 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,3 +1,4 @@
+extern crate bytecount;
 extern crate lalrpop_util;
 #[macro_use]
 extern crate nom;
@@ -11,16 +12,41 @@ pub mod parser;
 
 use std::io::{self, Read};
 
+fn print_error(message: &str, source: &str, start: usize, end: usize) {
+    let line_number = bytecount::count(source[..start].as_bytes(), '\n' as u8);
+    let line_start = source[..start].rfind('\n').map(|i| i + 1).unwrap_or(0);
+    let line_end = source[end..].find('\n').unwrap_or(0) + end;
+    let context = &source[line_start..line_end];
+
+    println!("error: {}", message);
+    println!("  --> (stdin):{}:0", line_number);
+    let line_number_str = line_number.to_string();
+    let line_number_width = line_number_str.len();
+    println!("{:w$} |", "", w = line_number_width);
+    println!("{} | {}", line_number_str, context);
+    println!("{:w$} | {:so$}{:^>ew$}", "", "", "",
+             w = line_number_width,
+             so = source[line_start..start].chars().count(),
+             ew = ::std::cmp::max(1, source[start..end].chars().count()));
+}
+
 fn main() {
     let mut buffer = String::new();
     io::stdin().read_to_string(&mut buffer).unwrap();
     match parser::parse_expr(&buffer) {
         Ok(e) => println!("{:?}", e),
         Err(lalrpop_util::ParseError::User { error: lexer::LexicalError::Error(pos, e) }) => {
-            let context = &buffer[pos..::std::cmp::min(buffer.len(), pos + 20)];
-            println!("Unexpected token in {:?}: {:?}...", e, context);
+            print_error(&format!("Unexpected token {:?}", e), &buffer, pos, pos);
+        }
+        Err(lalrpop_util::ParseError::UnrecognizedToken { token: Some((start, t, end)), expected: e }) => {
+            print_error(&format!("Unrecognized token {:?}", t), &buffer, start, end);
+            if e.len() > 0 {
+                println!("Expected {:?}", e);
+            }
+        }
+        Err(e) => {
+            print_error(&format!("Parser error {:?}", e), &buffer, 0, 0);
         }
-        Err(e) => println!("{:?}", e),
     }
 
     /*
-- 
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