Implement once nice error using annotate_snippets

2 files changed, 103 insertions, 0 deletions

diff --git a/dhall/src/error/builder.rs b/dhall/src/error/builder.rs
new file mode 100644
index 0000000..e293220
--- /dev/null
+++ b/dhall/src/error/builder.rs
@@ -0,0 +1,100 @@
+use annotate_snippets::{
+    display_list::DisplayList,
+    formatter::DisplayListFormatter,
+    snippet::{Annotation, AnnotationType, Slice, Snippet, SourceAnnotation},
+};
+
+use crate::syntax::Span;
+
+#[derive(Debug, Clone)]
+pub struct ErrorBuilder {
+    message: String,
+    annotation_type: AnnotationType,
+    annotations: Vec<BuilderAnnotation>,
+}
+
+#[derive(Debug, Clone)]
+struct BuilderAnnotation {
+    span: Span,
+    message: String,
+    annotation_type: AnnotationType,
+}
+
+/// A builder that uses the annotate_snippets library to display nice error messages about source
+/// code locations.
+impl ErrorBuilder {
+    pub fn new(message: String) -> Self {
+        ErrorBuilder {
+            message,
+            annotation_type: AnnotationType::Error,
+            annotations: Vec::new(),
+        }
+    }
+    pub fn span_err(span: &Span, message: String) -> Self {
+        let mut builder = Self::new(message.clone());
+        builder.annotate_err(span, message);
+        builder
+    }
+
+    pub fn annotate_err(&mut self, span: &Span, message: String) {
+        self.annotations.push(BuilderAnnotation {
+            span: span.clone(),
+            message,
+            annotation_type: AnnotationType::Error,
+        })
+    }
+    pub fn annotate_info(&mut self, span: &Span, message: String) {
+        self.annotations.push(BuilderAnnotation {
+            span: span.clone(),
+            message,
+            annotation_type: AnnotationType::Help,
+        })
+    }
+
+    // TODO: handle multiple files
+    pub fn format(self) -> String {
+        let mut input = None;
+        let annotations = self
+            .annotations
+            .into_iter()
+            .filter_map(|annot| {
+                let span = match annot.span {
+                    Span::Parsed(span) => span,
+                    _ => return None,
+                };
+                if input.is_none() {
+                    input = Some(span.to_input());
+                }
+                Some(SourceAnnotation {
+                    label: annot.message,
+                    annotation_type: annot.annotation_type,
+                    range: span.as_char_range(),
+                })
+            })
+            .collect();
+
+        let input = match input {
+            Some(input) => input,
+            None => return format!("[unknown location] {}", self.message),
+        };
+
+        let snippet = Snippet {
+            title: Some(Annotation {
+                label: Some(self.message),
+                id: None,
+                annotation_type: self.annotation_type,
+            }),
+            footer: vec![],
+            slices: vec![Slice {
+                source: input,
+                line_start: 1, // TODO
+                origin: Some("<current file>".to_string()),
+                fold: true,
+                annotations,
+            }],
+        };
+        let dl = DisplayList::from(snippet);
+        let dlf = DisplayListFormatter::new(true, false);
+        format!("{}", dlf.format(&dl))
+    }
+}
diff --git a/dhall/src/error/mod.rs b/dhall/src/error/mod.rs
index 4df018d..5b7693e 100644
--- a/dhall/src/error/mod.rs
+++ b/dhall/src/error/mod.rs
@@ -5,6 +5,9 @@ use crate::semantics::Value;
 use crate::syntax::{Import, ParseError};
 use crate::NormalizedExpr;
 
+mod builder;
+pub(crate) use builder::*;
+
 pub type Result<T> = std::result::Result<T, Error>;
 
 #[derive(Debug)]