compiler, cli: Support Nice Compilation Error Messages

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

Sources/Common/Error.swift

@@ -39,7 +39,29 @@ public struct Error: Errorable, Equatable, CustomStringConvertible {
   }
 }
 
+extension String {
+  public func trimmingPostfix(_ in_: String.Element) -> String {
+    return String(self.reversed().drop { $0 == in_ }.reversed())
+  }
+}
+
 public struct ErrorWithLocation: Errorable, Equatable, CustomStringConvertible {
+
+  public func format(_ formatter: any Formattable, _ sc: SourceCode) -> String {
+    guard let (fp, source, prior, after) = sc.getSourceSnippet(location: self.location, context: 5)
+    else {
+      return self.format(formatter)
+    }
+
+    let prior_snipped = prior.trimmingPrefix(["\n"])
+    let after_snipped = after.prefix { $0 != "\n" }
+    return formatter.formatWithStyle("Error: ", Style(StyleColor.Red))
+      + "In \(fp), there was an error: \n..." + prior_snipped
+      + formatter.formatWithStyle(source, Style(.none, [StyleFormat.Underline])) + after_snipped
+      + "...\n"
+      + self._msg
+  }
+
   public func format(_ formatter: any Formattable) -> String {
     let bold_red = Style(StyleColor.Red, [StyleFormat.Bold])
     let formatted_location = formatter.formatWithStyle(self.location.description, bold_red)

Sources/Common/Protocols.swift

@@ -72,6 +72,7 @@ public protocol ProgramExecutionEvaluator {
 
 public protocol Errorable: CustomStringConvertible {
   func format(_ formatter: Formattable) -> String
+  func format(_ formatter: Formattable, _ sc: SourceCode) -> String
   func msg() -> String
   func append(error: any Errorable) -> any Errorable
   func eq(_ rhs: any Errorable) -> Bool
@@ -81,6 +82,9 @@ extension Errorable {
   public func eq(_ rhs: any Errorable) -> Bool {
     return self.msg() == rhs.msg()
   }
+  public func format(_ formatter: Formattable, _ sc: SourceCode) -> String {
+    return self.format(formatter)
+  }
 }
 
 public protocol Formattable {

Sources/Common/SourceCode.swift

@@ -210,6 +210,33 @@ public struct SourceCode {
     return self.code
   }
 
+  public func getSourceSnippet(
+    location: SourceLocation, context: Int = 0
+  ) -> (FilePath, String, String, String)? {
+    guard let path = self.pathForLocation(location.range.lowerBound) else {
+      return .none
+    }
+    let lower = String.UTF8View.Index(utf16Offset: location.range.lowerBound, in: self.code)
+    let upper = String.UTF8View.Index(utf16Offset: location.range.upperBound, in: self.code)
+    let prior_start =
+      if location.range.lowerBound - context >= 0 {
+        String.UTF8View.Index(utf16Offset: location.range.lowerBound - context, in: self.code)
+      } else {
+        String.UTF8View.Index(utf16Offset: location.range.lowerBound, in: self.code)
+      }
+    let after_end =
+      if location.range.upperBound + context < self.code.count {
+        String.UTF8View.Index(utf16Offset: location.range.upperBound + context, in: self.code)
+      } else {
+        String.UTF8View.Index(utf16Offset: location.range.upperBound, in: self.code)
+      }
+
+    let result = String(self.code.utf16[lower..<upper])!
+    let prior = String(self.code.utf16[prior_start..<lower])!
+    let after = String(self.code.utf16[upper...after_end])!
+    return (path, result, prior, after)
+  }
+
   public func getLocations() -> FileSourceLocation {
     return self.locations
   }