compiler, cli: Support Nice Compilation Error Messages

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

Sources/Cli/main.swift

@@ -96,7 +96,8 @@ extension Cli {
 
       let maybe_program = Program.Compile(source.getSource())
       guard case .Ok(_) = maybe_program else {
-        print(ErrorWithLabel("Compiler Error", maybe_source.error()!).format(formatter))
+        let feedback = CompilationFeedback(source, [maybe_program.error()!], formatter)
+        print(feedback.feedback)
         return
       }
 

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
   }

Sources/P4Compiler/Feedback.swift

@@ -0,0 +1,33 @@
+// p4rse, Copyright 2026, Will Hawkins
+//
+// This file is part of p4rse.
+//
+// This file is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+import Common
+
+public struct CompilationFeedback {
+  public let feedback: String
+
+  static func markup(
+    _ source: SourceCode, _ errors: [any Errorable], _ formatter: any Formattable
+  ) -> String {
+    errors.map { $0.format(formatter, source) }.joined(separator: "\n")
+  }
+
+  public init(_ source: SourceCode, _ errors: [any Errorable], _ formatter: any Formattable) {
+    self.feedback = Self.markup(source, errors, formatter)
+  }
+
+}

TestData/Sources/action-parameters-wrong-order.p4

@@ -0,0 +1,13 @@
+    control simple(int x, int y) {
+      action a(inout bool aa, int ax, inout bool ay) {
+        aa = false;
+      }
+      table t {
+        key = {
+          x: exact;
+          y: exact;
+        }
+      }
+      apply {
+      }
+    };

TestData/simple_cli_compilation_error.golden

@@ -0,0 +1,3 @@
+Error: In /Users/hawkinsw/code/p4ce/TestData/Sources/action-parameters-wrong-order.p4, there was an error: 
+...ion a(inout bool aa, int ax, inout bool ay) {...
+All parameters with direction must precede directionless parameters

Tests/p4rseTests/CliTests/Cli.swift

@@ -38,4 +38,9 @@ func simple_cli_preprocessor_test() -> [String] {
 @CliTest()
 func simple_cli_preprocessor_test_file_not_found() -> [String] {
     return ["p4ce", "--plain", "preprocess", "simple.p", "-I", "TestData/Sources/"]
+}
+
+@CliTest()
+func simple_cli_compilation_error() -> [String] {
+    return ["p4ce", "--plain", "compile", "action-parameters-wrong-order.p4", "-I", "TestData/Sources/"]
 }

Tests/p4rseTests/ControlTests/Compile.swift

@@ -136,7 +136,8 @@ import TreeSitterP4
   #expect(
     #RequireErrorResult(
       Error(
-        withMessage: "{54, 63}: Error(s) parsing property list: {91, 26}: Error(s) parsing table actions: Cannot find b in lexical scope."
+        withMessage:
+          "{54, 63}: Error(s) parsing property list: {91, 26}: Error(s) parsing table actions: Cannot find b in lexical scope."
       ),
       Program.Compile(simple_parser_declaration))
   )
@@ -163,7 +164,8 @@ import TreeSitterP4
   #expect(
     #RequireErrorResult(
       Error(
-        withMessage: "{54, 72}: Error(s) parsing property list: {91, 35}: Error(s) parsing table actions: Cannot find b in lexical scope."
+        withMessage:
+          "{54, 72}: Error(s) parsing property list: {91, 35}: Error(s) parsing table actions: Cannot find b in lexical scope."
       ),
       Program.Compile(simple_parser_declaration))
   )
@@ -190,7 +192,8 @@ import TreeSitterP4
   #expect(
     #RequireErrorResult(
       Error(
-        withMessage: "{64, 63}: Error(s) parsing property list: {101, 26}: Error(s) parsing table actions: {101, 26}: a does not name an action"
+        withMessage:
+          "{64, 63}: Error(s) parsing property list: {101, 26}: Error(s) parsing table actions: {101, 26}: a does not name an action"
       ),
       Program.Compile(simple_parser_declaration))
   )