From 6c9a197b37d38e650e9b86fbf54351d252a3029c Mon Sep 17 00:00:00 2001
From: Krasimir Angelov <kr.angelov@gmail.com>
Date: Fri, 14 Nov 2025 12:05:11 +0100
Subject: [PATCH] more helpful error when lin X is missing

---
 src/compiler/api/GF/Compile/SubExOpt.hs           |  3 ++-
 src/compiler/api/GF/Compile/TypeCheck/Concrete.hs | 13 ++++++++++---
 src/compiler/api/GF/Grammar/Lockfield.hs          |  4 ++--
 src/compiler/api/GF/Infra/Ident.hs                |  6 ++++--
 4 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/src/compiler/api/GF/Compile/SubExOpt.hs b/src/compiler/api/GF/Compile/SubExOpt.hs
index 09ec3e568..6b9d908b9 100644
--- a/src/compiler/api/GF/Compile/SubExOpt.hs
+++ b/src/compiler/api/GF/Compile/SubExOpt.hs
@@ -31,6 +31,7 @@ import qualified GF.Grammar.Macros as C
 import GF.Data.ErrM(fromErr)
 
 import Control.Monad.State.Strict(State,evalState,get,put)
+import Data.Maybe(isJust)
 import Data.Map (Map)
 import qualified Data.Map as Map
 
@@ -136,6 +137,6 @@ operIdent :: Int -> Ident
 operIdent i = identC (operPrefix `prefixRawIdent` (rawIdentS (show i))) ---
 
 isOperIdent :: Ident -> Bool
-isOperIdent id = isPrefixOf operPrefix (ident2raw id)
+isOperIdent id = isJust (isPrefixOf operPrefix (ident2raw id))
 
 operPrefix = rawIdentS ("A''")
diff --git a/src/compiler/api/GF/Compile/TypeCheck/Concrete.hs b/src/compiler/api/GF/Compile/TypeCheck/Concrete.hs
index f51798215..80c71ac8c 100644
--- a/src/compiler/api/GF/Compile/TypeCheck/Concrete.hs
+++ b/src/compiler/api/GF/Compile/TypeCheck/Concrete.hs
@@ -6,6 +6,7 @@ module GF.Compile.TypeCheck.Concrete ( checkLType, checkLType', inferLType, infe
 -- Practical type inference for arbitrary-rank types.
 -- 14 September 2011
 
+import Prelude hiding ((<>))
 import GF.Grammar hiding (Env, VGen, VApp, VRecType, ppValue)
 import GF.Grammar.Lookup
 import GF.Grammar.Predef
@@ -1083,15 +1084,21 @@ subsCheckRho scope t ty1@(VRecType rs1 ext1) ty2@(VRecType rs2 ext2) = do      -
       mkField scope l (mb_ty,t) (Just ty1) ty2 = do
         (t,ty1,ty2) <- subsCheckRho scope t ty1 ty2
         return ((l, (mb_ty,t)), (l, True, ty1))
-      mkField scope l (mb_ty,t) Nothing    ty2
-        | isLockLabel l = return ((l, (Just (RecType []),R [])), (l, True, ty2))
-        | otherwise     = return ((l, (mb_ty,t)), (l, True, ty2))
+      mkField scope l (mb_ty,t) Nothing    ty2 =
+        case isLockLabel l of
+          Just _  -> return ((l, (Just (RecType []),R [])), (l, True, ty2))
+          Nothing -> return ((l, (mb_ty,t)), (l, True, ty2))
 
   (scope,mkProj,wrap) <- mkAccess scope t
 
   let fields = [(l,o2,ty2,lookup3 l rs1) | (l,o2,ty2) <- rs2]
   case [l | (l,_,_,Nothing) <- fields, not ext1] of
     []      -> return ()
+    [field] -> evalError ("In the term" <+> pp t $$
+                          "there is no value for field" <+> field <>
+                          case isLockLabel field of
+                            Just cat -> ", try wrapping with lin"<+>pp cat
+                            Nothing  -> empty)
     missing -> evalError ("In the term" <+> pp t $$
                           "there are no values for fields:" <+> hsep missing)
   rs <- sequence [mkField scope l t mb_ty1 ty2 | (l,_,ty2,mb_ty1) <- fields, Just t <- [mkProj l]]
diff --git a/src/compiler/api/GF/Grammar/Lockfield.hs b/src/compiler/api/GF/Grammar/Lockfield.hs
index 9eb487723..cab066811 100644
--- a/src/compiler/api/GF/Grammar/Lockfield.hs
+++ b/src/compiler/api/GF/Grammar/Lockfield.hs
@@ -40,9 +40,9 @@ lock c t           = t
 lockLabel :: Ident -> Label
 lockLabel c = LIdent $! prefixRawIdent lockPrefix (ident2raw c)
 
-isLockLabel :: Label -> Bool
+isLockLabel :: Label -> Maybe RawIdent
 isLockLabel l = case l of
   LIdent c -> isPrefixOf lockPrefix c
-  _        -> False
+  _        -> Nothing
 
 lockPrefix = rawIdentS "lock_"
diff --git a/src/compiler/api/GF/Infra/Ident.hs b/src/compiler/api/GF/Infra/Ident.hs
index e202512f4..fbad6e694 100644
--- a/src/compiler/api/GF/Infra/Ident.hs
+++ b/src/compiler/api/GF/Infra/Ident.hs
@@ -26,7 +26,7 @@ module GF.Infra.Ident (-- ** Identifiers
 ) where
 
 import qualified Data.ByteString.UTF8 as UTF8
-import qualified Data.ByteString.Char8 as BS(append,isPrefixOf)
+import qualified Data.ByteString.Char8 as BS(append,isPrefixOf,drop,length)
                  -- Limit use of BS functions to the ones that work correctly on
                  -- UTF-8-encoded bytestrings!
 import Data.Char(isDigit)
@@ -75,7 +75,9 @@ rawIdentC = Id
 showRawIdent = unpack . rawId2utf8
 
 prefixRawIdent (Id x) (Id y) = Id (BS.append x y)
-isPrefixOf (Id x) (Id y) = BS.isPrefixOf x y
+isPrefixOf (Id x) (Id y)
+ | BS.isPrefixOf x y = Just (Id (BS.drop (BS.length x) y))
+ | otherwise         = Nothing
 
 instance Binary Ident where
   put id = put (ident2utf8 id)