compiler: @FILE, @LINE, @FN, @COLUMN

compiler/comptime.v

@@ -115,7 +115,7 @@ fn (p mut Parser) comp_time() {
 			os.rm('.vwebtmpl.v')
 		}
 		pp.is_vweb = true
-		pp.cur_fn = p.cur_fn // give access too all variables in current function
+		pp.set_current_fn( p.cur_fn ) // give access too all variables in current function
 		pp.parse(.main)
 		tmpl_fn_body := p.cgen.lines.slice(pos + 2, p.cgen.lines.len).join('\n').clone()
 		end_pos := tmpl_fn_body.last_index('Builder_str( sb )')  + 19 // TODO

compiler/fn.v

@@ -284,7 +284,7 @@ fn (p mut Parser) fn_decl() {
 		''
 	}
 	if !p.is_vweb {
-		p.cur_fn = f
+		p.set_current_fn( f )
 	}
 	// Generate `User_register()` instead of `register()`
 	// Internally it's still stored as "register" in type User
@@ -492,7 +492,7 @@ _thread_so = CreateThread(0, 0, (LPTHREAD_START_ROUTINE)&reload_so, 0, 0, 0);
 		return
 	}
 	p.check_unused_variables()
-	p.cur_fn = EmptyFn
+	p.set_current_fn( EmptyFn )
 	p.returns = false
 	if !is_generic {
 		p.genln('}')

compiler/parser.v

@@ -119,6 +119,11 @@ fn (v mut V) new_parser(path string) Parser {
 	return p
 }
 
+fn (p mut Parser) set_current_fn(f &Fn) {
+	p.cur_fn = f
+	p.scanner.fn_name = '${f.mod}.${f.name}'
+}
+
 fn (p mut Parser) next() {
 	p.prev_tok2 = p.prev_tok
 	p.prev_tok = p.tok
@@ -262,7 +267,7 @@ fn (p mut Parser) parse(pass Pass) {
 		case Token.eof:
 			p.log('end of parse()')
 			if p.is_script && !p.pref.is_test {
-				p.cur_fn = MainFn
+				p.set_current_fn( MainFn )
 				p.check_unused_variables()
 			}
 			if false && !p.first_pass() && p.fileis('main.v') {
@@ -281,12 +286,12 @@ fn (p mut Parser) parse(pass Pass) {
 				// we need to set it to save and find variables
 				if p.first_pass() {
 					if p.cur_fn.name == '' {
-						p.cur_fn = MainFn
+						p.set_current_fn( MainFn )
 					}
 					return
 				}
 				if p.cur_fn.name == '' {
-					p.cur_fn = MainFn
+					p.set_current_fn( MainFn )
 					if p.pref.is_repl {
 						p.cur_fn.clear_vars()
 					}
@@ -2532,16 +2537,28 @@ fn (p mut Parser) string_expr() {
 		// Custom format? ${t.hour:02d}
 		custom := p.tok == .colon
 		if custom {
-			format += '%'
+			mut cformat := ''
 			p.next()
 			if p.tok == .dot {
-				format += '.'
+				cformat += '.'
 				p.next()
 			}
-			format += p.lit// 02
+			if p.tok == .minus { // support for left aligned formatting
+				cformat += '-'
+				p.next()
+			}
+			cformat += p.lit// 02
 			p.next()
-			format += p.lit// f
+			fspec := p.lit // f
-			// println('custom str F=$format')
+			cformat += fspec
+			if fspec == 's' {
+				//println('custom str F=$cformat | format_specifier: "$fspec" | typ: $typ ')
+				if typ != 'string' {
+					p.error('only v strings can be formatted with a :${cformat} format, but you have given "${val}", which has type ${typ}.')
+				}
+				args = args.all_before_last('${val}.len, ${val}.str') + '${val}.str'
+			}
+			format += '%$cformat'
 			p.next()
 		}
 		else {
@@ -2563,6 +2580,7 @@ fn (p mut Parser) string_expr() {
 			}
 			format += f
 		}
+		//println('interpolation format is: |${format}| args are: |${args}| ')
 	}
 	if complex_inter {
 		p.fgen('}')

compiler/scanner.v

@@ -24,6 +24,7 @@ mut:
 	fmt_indent     int
 	fmt_line_empty bool
 	prev_tok Token
+	fn_name string // needed for @FN
 }
 
 fn new_scanner(file_path string) &Scanner {
@@ -394,6 +395,17 @@ fn (s mut Scanner) scan() ScanRes {
 	case `@`:
 		s.pos++
 		name := s.ident_name()
+		// @FN => will be substituted with the name of the current V function
+		// @FILE => will be substituted with the path of the V source file
+		// @LINE => will be substituted with the V line number where it appears (as a string).
+		// @COLUMN => will be substituted with the column where it appears (as a string).
+		// This allows things like this: 
+		// println( 'file: ' + @FILE + ' | line: ' + @LINE + ' | fn: ' + @FN)
+		// ... which is useful while debugging/tracing
+		if name == 'FN' { return scan_res(.str, s.fn_name) }
+		if name == 'FILE' { return scan_res(.str, os.realpath(s.file_path)) }
+		if name == 'LINE' { return scan_res(.str, (s.line_nr+1).str()) }
+		if name == 'COLUMN' { return scan_res(.str, (s.current_column()).str()) }
 		if !is_key(name) {
 			s.error('@ must be used before keywords (e.g. `@type string`)')
 		}
@@ -580,9 +592,13 @@ fn (s &Scanner) find_current_line_start_position() int {
   return linestart
 }
 
+fn (s &Scanner) current_column() int {
+	return s.pos - s.find_current_line_start_position()
+}
+
 fn (s &Scanner) error(msg string) {
 	fullpath := os.realpath( s.file_path )
-	column := s.pos - s.find_current_line_start_position()
+	column := s.current_column()
 	// The filepath:line:col: format is the default C compiler
 	// error output format. It allows editors and IDE's like
 	// emacs to quickly find the errors in the output