Merge pull request #114 from Nadrieril/nice-type-errors

Ground work for pretty type errors

4 files changed, 92 insertions, 58 deletions

diff --git a/dhall_syntax/src/core/expr.rs b/dhall_syntax/src/core/expr.rs
index 2ad3ba8..750b58b 100644
--- a/dhall_syntax/src/core/expr.rs
+++ b/dhall_syntax/src/core/expr.rs
@@ -1,5 +1,3 @@
-use std::rc::Rc;
-
 use crate::map::{DupTreeMap, DupTreeSet};
 use crate::visitor::{self, ExprFMutVisitor, ExprFVisitor};
 use crate::*;
@@ -15,40 +13,6 @@ pub fn trivial_result<T>(x: Result<T, !>) -> T {
     }
 }
 
-/// A location in the source text
-#[derive(Debug, Clone)]
-pub struct Span {
-    input: Rc<str>,
-    /// # Safety
-    ///
-    /// Must be a valid character boundary index into `input`.
-    start: usize,
-    /// # Safety
-    ///
-    /// Must be a valid character boundary index into `input`.
-    end: usize,
-}
-
-impl Span {
-    pub(crate) fn make(input: Rc<str>, sp: pest::Span) -> Self {
-        Span {
-            input,
-            start: sp.start(),
-            end: sp.end(),
-        }
-    }
-    /// Takes the union of the two spans. Assumes that the spans come from the same input.
-    /// This will also capture any input between the spans.
-    pub fn union(&self, other: &Span) -> Self {
-        use std::cmp::{max, min};
-        Span {
-            input: self.input.clone(),
-            start: min(self.start, other.start),
-            end: max(self.start, other.start),
-        }
-    }
-}
-
 /// Double with bitwise equality
 #[derive(Debug, Copy, Clone)]
 pub struct NaiveDouble(f64);
@@ -178,7 +142,7 @@ pub enum Builtin {
 
 // Each node carries an annotation.
 #[derive(Debug, Clone)]
-pub struct Expr<Embed>(Box<(RawExpr<Embed>, Option<Span>)>);
+pub struct Expr<Embed>(Box<(RawExpr<Embed>, Span)>);
 
 pub type RawExpr<Embed> = ExprF<Expr<Embed>, Embed>;
 
@@ -334,16 +298,12 @@ impl<E> Expr<E> {
     pub fn as_mut(&mut self) -> &mut RawExpr<E> {
         &mut self.0.as_mut().0
     }
-    pub fn span(&self) -> Option<Span> {
+    pub fn span(&self) -> Span {
         self.0.as_ref().1.clone()
     }
 
-    pub(crate) fn new(x: RawExpr<E>, n: Span) -> Self {
-        Expr(Box::new((x, Some(n))))
-    }
-
-    pub fn from_expr_no_span(x: RawExpr<E>) -> Self {
-        Expr(Box::new((x, None)))
+    pub fn new(x: RawExpr<E>, n: Span) -> Self {
+        Expr(Box::new((x, n)))
     }
 
     pub fn rewrap<E2>(&self, x: RawExpr<E2>) -> Expr<E2> {
@@ -388,11 +348,6 @@ impl<E> Expr<E> {
     }
 }
 
-// Should probably rename this
-pub fn rc<E>(x: RawExpr<E>) -> Expr<E> {
-    Expr::from_expr_no_span(x)
-}
-
 /// Add an isize to an usize
 /// Panics on over/underflow
 fn add_ui(u: usize, i: isize) -> Option<usize> {
diff --git a/dhall_syntax/src/core/mod.rs b/dhall_syntax/src/core/mod.rs
index fe2c0be..66bf229 100644
--- a/dhall_syntax/src/core/mod.rs
+++ b/dhall_syntax/src/core/mod.rs
@@ -4,6 +4,8 @@ mod import;
 pub use import::*;
 mod label;
 pub use label::*;
+mod span;
+pub use span::*;
 mod text;
 pub use text::*;
 pub mod context;
diff --git a/dhall_syntax/src/core/span.rs b/dhall_syntax/src/core/span.rs
new file mode 100644
index 0000000..f9c7008
--- /dev/null
+++ b/dhall_syntax/src/core/span.rs
@@ -0,0 +1,81 @@
+use std::rc::Rc;
+
+/// A location in the source text
+#[derive(Debug, Clone)]
+pub struct ParsedSpan {
+    input: Rc<str>,
+    /// # Safety
+    ///
+    /// Must be a valid character boundary index into `input`.
+    start: usize,
+    /// # Safety
+    ///
+    /// Must be a valid character boundary index into `input`.
+    end: usize,
+}
+
+#[derive(Debug, Clone)]
+pub enum Span {
+    /// A location in the source text
+    Parsed(ParsedSpan),
+    /// For expressions obtained from decoding binary
+    Decoded,
+    /// For expressions constructed during normalization/typecheck
+    Artificial,
+}
+
+impl Span {
+    pub(crate) fn make(input: Rc<str>, sp: pest::Span) -> Self {
+        Span::Parsed(ParsedSpan {
+            input,
+            start: sp.start(),
+            end: sp.end(),
+        })
+    }
+
+    /// Takes the union of the two spans, i.e. the range of input covered by the two spans plus any
+    /// input between them. Assumes that the spans come from the same input. Fails if one of the
+    /// spans does not point to an input location.
+    pub fn union(&self, other: &Span) -> Self {
+        use std::cmp::{max, min};
+        use Span::*;
+        match (self, other) {
+            (Parsed(x), Parsed(y)) if Rc::ptr_eq(&x.input, &y.input) => {
+                Parsed(ParsedSpan {
+                    input: x.input.clone(),
+                    start: min(x.start, y.start),
+                    end: max(x.end, y.end),
+                })
+            }
+            _ => panic!(
+                "Tried to union incompatible spans: {:?} and {:?}",
+                self, other
+            ),
+        }
+    }
+
+    /// Merges two spans assumed to point to a similar thing. If only one of them points to an
+    /// input location, use that one.
+    pub fn merge(&self, other: &Span) -> Self {
+        use Span::*;
+        match (self, other) {
+            (Parsed(x), _) | (_, Parsed(x)) => Parsed(x.clone()),
+            (Artificial, _) | (_, Artificial) => Artificial,
+            (Decoded, Decoded) => Decoded,
+        }
+    }
+
+    pub fn error(&self, message: impl Into<String>) -> String {
+        use pest::error::{Error, ErrorVariant};
+        use pest::Span;
+        let message: String = message.into();
+        let span = match self {
+            self::Span::Parsed(span) => span,
+            _ => return format!("[unknown location] {}", message),
+        };
+        let span = Span::new(&*span.input, span.start, span.end).unwrap();
+        let err: ErrorVariant<!> = ErrorVariant::CustomError { message };
+        let err = Error::new_from_span(err, span);
+        format!("{}", err)
+    }
+}
diff --git a/dhall_syntax/src/parser.rs b/dhall_syntax/src/parser.rs
index 83f62ee..eaded50 100644
--- a/dhall_syntax/src/parser.rs
+++ b/dhall_syntax/src/parser.rs
@@ -650,7 +650,7 @@ impl DhallParser {
                     final_expr,
                     |acc, x| {
                         spanned_union(
-                            acc.span().unwrap(),
+                            acc.span(),
                             x.3,
                             Let(x.0, x.1, x.2, acc)
                         )
@@ -717,11 +717,7 @@ impl DhallParser {
             r => Err(op.error(format!("Rule {:?} isn't an operator", r)))?,
         };
 
-        Ok(spanned_union(
-            l.span().unwrap(),
-            r.span().unwrap(),
-            BinOp(op, l, r),
-        ))
+        Ok(spanned_union(l.span(), r.span(), BinOp(op, l, r)))
     }
 
     fn Some_(_input: ParseInput) -> ParseResult<()> {
@@ -739,8 +735,8 @@ impl DhallParser {
                     first,
                     |acc, e| {
                         spanned_union(
-                            acc.span().unwrap(),
-                            e.span().unwrap(),
+                            acc.span(),
+                            e.span(),
                             App(acc, e)
                         )
                     }
@@ -778,7 +774,7 @@ impl DhallParser {
                     first,
                     |acc, e| {
                         spanned_union(
-                            acc.span().unwrap(),
+                            acc.span(),
                             e.1,
                             match e.0 {
                                 Either::Left(l) => Field(acc, l),