From afca46a45e9e1bd5dbed53a50587e044eacc1d08 Mon Sep 17 00:00:00 2001
From: Sebastian Graf <sebastian.graf@kit.edu>
Date: Wed, 10 Jan 2024 13:10:37 +0100
Subject: [PATCH] Parser: Add a Note detailing why we need happy's `error` to
implement layout
---
compiler/GHC/Parser.y | 33 ++++++++++++++++++++++++++++++++-
1 file changed, 32 insertions(+), 1 deletion(-)
diff --git a/compiler/GHC/Parser.y b/compiler/GHC/Parser.y
index 2257f55603fb..6750ee526132 100644
--- a/compiler/GHC/Parser.y
+++ b/compiler/GHC/Parser.y
@@ -3943,9 +3943,40 @@ literal :: { Located (HsLit GhcPs) }
-----------------------------------------------------------------------------
-- Layout
+{- Note [Layout and error]
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+The Haskell 2010 report (Section 10.3, Note 5) dictates the use of the error
+token in `close`. To recall why that is necessary, consider
+
+ f x = case x of
+ True -> False
+ where y = x+1
+
+The virtual pass L inserts vocurly, semi, vccurly to return a laid-out
+token stream. It must insert a vccurly before `where` to close the layout
+block introduced by `of`.
+But there is no good way to do so other than L becoming aware of the grammar!
+Thus, L is specified to detect the ensuing parse error (implemented via
+happy's `error` token) and then insert the vccurly.
+Thus in effect, L is distributed between Lexer.x and Parser.y.
+
+There are a bunch of other, less "tricky" examples:
+
+ let x = x {- vccurly -} in x -- could just track bracketing of
+ -- let..in on layout stack to fix
+ (case x of
+ True -> False {- vccurly -}) -- ditto for surrounding delimiters such as ()
+
+ data T = T;{- vccurly -} -- Need insert vccurly at EOF
+
+Many of these are not that hard to fix, but still tedious and prone to break
+when the grammar changes; but the `of`/`where` example is especially gnarly,
+because it demonstrates a grammatical interaction between two lexically
+unrelated tokens.
+-}
close :: { () }
: vccurly { () } -- context popped in lexer.
- | error {% popContext }
+ | error {% popContext } -- See Note [Layout and error]
-----------------------------------------------------------------------------
-- Miscellaneous (mostly renamings)
--
GitLab