add 'error contexts' to parser error reports

examples/hello.ry

@@ -1,4 +1,4 @@
-import std;
+// import std;
 
 fn add(a: i32, b: i32) i32 {
     return a + b;

src/errors.zig

@@ -1,15 +1,24 @@
 const std = @import("std");
 
-pub fn report(line: usize, where: []const u8, message: []const u8) void {
+pub fn report(line: usize, where: []const u8, ctx_opt: ?[]const u8, message: []const u8) void {
+    if (ctx_opt) |ctx| {
+        std.debug.warn("[line {}] Error{} on {}: {}", line, where, ctx, message);
+    } else {
         std.debug.warn("[line {}] Error{}: {}", line, where, message);
+    }
 }
 
 pub fn reportN(line: usize, message: []const u8) void {
     report(line, "", message);
 }
 
-pub fn reportFmt(line: usize, comptime fmt: []const u8, args: ...) void {
+pub fn reportFmt(line: usize, ctx_opt: ?[]const u8, comptime fmt: []const u8, args: ...) void {
+    if (ctx_opt) |ctx| {
+        std.debug.warn("[line {}] Error on {}", line, ctx);
+    } else {
         std.debug.warn("[line {}] Error", line);
+    }
+
     std.debug.warn(fmt, args);
     std.debug.warn("\n");
 }

src/parsers.zig

@@ -31,6 +31,7 @@ pub const Parser = struct {
     tokens: TokenList,
 
     hadError: bool = false,
+    err_ctx: ?[]const u8 = null,
 
     pub fn init(allocator: *Allocator, scanner: *Scanner) Parser {
         return Parser{
@@ -44,10 +45,26 @@ pub const Parser = struct {
         self.tokens.deinit();
     }
 
+    fn setErrContext(self: *Parser, comptime fmt: ?[]const u8, args: ...) void {
+        if (fmt == null) {
+            self.err_ctx = null;
+            return;
+        }
+
+        var buf = self.allocator.alloc(u8, 256) catch unreachable;
+        self.err_ctx = std.fmt.bufPrint(buf, fmt.?, args) catch unreachable;
+    }
+
     fn doError(self: *Parser, comptime fmt: []const u8, args: ...) ParseError {
         self.hadError = true;
 
-        std.debug.warn("parser error at line {}\n\t", self.scanner.line);
+        std.debug.warn("parser error at line {}", self.scanner.line);
+        if (self.err_ctx) |ctx| {
+            std.debug.warn(" on {}", ctx);
+        }
+
+        std.debug.warn("\n\t");
+
         std.debug.warn(fmt, args);
         std.debug.warn("\n");
 
@@ -80,10 +97,11 @@ pub const Parser = struct {
     }
 
     fn tokenError(self: *Parser, token: Token, msg: []const u8) ParseError {
+        std.debug.warn("ctx: '{}'\n", self.err_ctx);
         if (token.typ == .EOF) {
-            ereport.report(token.line, " at end", msg);
+            ereport.report(token.line, " at end", self.err_ctx, msg);
         } else {
-            ereport.reportFmt(token.line, " at '{}': {}", token.lexeme, msg);
+            ereport.reportFmt(token.line, self.err_ctx, " at '{}': {}", token.lexeme, msg);
         }
 
         return ParseError.CompileError;
@@ -439,6 +457,8 @@ pub const Parser = struct {
 
         const name = try self.consumeSingle(.Identifier);
 
+        self.setErrContext("function {}", name.lexeme);
+
         _ = try self.consumeSingle(.LeftParen);
 
         while (self.peek().typ != .RightParen) {
@@ -623,6 +643,8 @@ pub const Parser = struct {
     }
 
     fn parseTopDecl(self: *@This()) !*Node {
+        self.setErrContext(null);
+
         return switch (self.peek().typ) {
             .Fn => try self.parseFnDecl(),
             .Const => try self.parseConstDecl(),