diff --git a/src/libexpr/lexer-helpers.cc b/src/libexpr/lexer-helpers.cc
new file mode 100644
index 00000000000..87c514c71cc
--- /dev/null
+++ b/src/libexpr/lexer-helpers.cc
@@ -0,0 +1,31 @@
+#include "lexer-tab.hh"
+#include "lexer-helpers.hh"
+#include "parser-tab.hh"
+
+void nix::lexer::internal::initLoc(YYLTYPE * loc)
+{
+    loc->first_line = loc->last_line = 0;
+    loc->first_column = loc->last_column = 0;
+}
+
+void nix::lexer::internal::adjustLoc(yyscan_t yyscanner, YYLTYPE * loc, const char * s, size_t len)
+{
+    loc->stash();
+
+    LexerState & lexerState = *yyget_extra(yyscanner);
+
+    if (lexerState.docCommentDistance == 1) {
+        // Preceding token was a doc comment.
+        ParserLocation doc;
+        doc.first_column = lexerState.lastDocCommentLoc.first_column;
+        ParserLocation docEnd;
+        docEnd.first_column = lexerState.lastDocCommentLoc.last_column;
+        DocComment docComment{lexerState.at(doc), lexerState.at(docEnd)};
+        PosIdx locPos = lexerState.at(*loc);
+        lexerState.positionToDocComment.emplace(locPos, docComment);
+    }
+    lexerState.docCommentDistance++;
+
+    loc->first_column = loc->last_column;
+    loc->last_column += len;
+}
diff --git a/src/libexpr/lexer-helpers.hh b/src/libexpr/lexer-helpers.hh
new file mode 100644
index 00000000000..caba6e18f48
--- /dev/null
+++ b/src/libexpr/lexer-helpers.hh
@@ -0,0 +1,9 @@
+#pragma once
+
+namespace nix::lexer::internal {
+
+void initLoc(YYLTYPE * loc);
+
+void adjustLoc(yyscan_t yyscanner, YYLTYPE * loc, const char * s, size_t len);
+
+} // namespace nix::lexer
diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index d6777b59544..a016a63d586 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -14,6 +14,10 @@
 %x INPATH_SLASH
 %x PATH_START
 
+%top {
+#include "parser-tab.hh" // YYSTYPE
+#include "parser-state.hh"
+}
 
 %{
 #ifdef __clang__
@@ -22,48 +26,19 @@
 
 #include "nixexpr.hh"
 #include "parser-tab.hh"
+#include "lexer-helpers.hh"
 
-// !!! FIXME !!!
-#define YY_EXTRA_TYPE ::nix::LexerState *
-int yylex_init_extra ( YY_EXTRA_TYPE user_defined, yyscan_t* scanner);
-YY_EXTRA_TYPE yyget_extra ( yyscan_t yyscanner );
-#undef YY_EXTRA_TYPE
+namespace nix {
+  struct LexerState;
+}
 
 using namespace nix;
+using namespace nix::lexer::internal;
 
 namespace nix {
 
 #define CUR_POS state->at(*yylloc)
 
-static void initLoc(YYLTYPE * loc)
-{
-    loc->first_line = loc->last_line = 0;
-    loc->first_column = loc->last_column = 0;
-}
-
-static void adjustLoc(yyscan_t yyscanner, YYLTYPE * loc, const char * s, size_t len)
-{
-    loc->stash();
-
-    LexerState & lexerState = *yyget_extra(yyscanner);
-
-    if (lexerState.docCommentDistance == 1) {
-        // Preceding token was a doc comment.
-        ParserLocation doc;
-        doc.first_column = lexerState.lastDocCommentLoc.first_column;
-        ParserLocation docEnd;
-        docEnd.first_column = lexerState.lastDocCommentLoc.last_column;
-        DocComment docComment{lexerState.at(doc), lexerState.at(docEnd)};
-        PosIdx locPos = lexerState.at(*loc);
-        lexerState.positionToDocComment.emplace(locPos, docComment);
-    }
-    lexerState.docCommentDistance++;
-
-    loc->first_column = loc->last_column;
-    loc->last_column += len;
-}
-
-
 // we make use of the fact that the parser receives a private copy of the input
 // string and can munge around in it.
 static StringToken unescapeStr(SymbolTable & symbols, char * s, size_t length)