Confusing "parse error in pattern" for spurious indentation.
The following problem exists at least in ghc 7.4.2 through 7.10.1:
main = do
putStrLn "Hello, lots of crap" $ do
a <- return 3
-- The following line is mis-indented, the error is incomprehensible
c <- do
return 5
Error:
<file>:2:3:
Parse error in pattern: putStrLn
Possibly caused by a missing 'do'?
The hint about "missing do" exists from ghc 7.8, but is wrong in this case. Problematic about this bug is that the wrong indentation can be arbitrary many lines from where ghc reports the (totally incomprehensible) error. My original problem is to find the syntax error here:
-- | Type check a function clause.
checkClause :: Type -> A.SpineClause -> TCM Clause
checkClause t c@(A.Clause (A.SpineLHS i x aps withPats) rhs0 wh) = do
unless (null withPats) $ do
typeError $ UnexpectedWithPatterns withPats
traceCall (CheckClause t c) $ do
aps <- expandPatternSynonyms aps
checkLeftHandSide (CheckPatternShadowing c) (Just x) aps t $
\ (LHSResult delta ps trhs perm) -> do
-- Note that we might now be in irrelevant context,
-- in case checkLeftHandSide walked over an irrelevant projection pattern.
-- As we will be type-checking the @rhs@ in @delta@, but the final
-- body should have bindings in the order of the pattern variables,
-- we need to apply the permutation to the checked rhs @v@.
let mkBody v = foldr (\ x t -> Bind $ Abs x t) b xs
where b = Body $ applySubst (renamingR perm) v
xs = [ stringToArgName $ "h" ++ show n
| n <- [0..permRange perm - 1] ]
-- introduce trailing implicits for checking the where decls
TelV htel t0 <- telViewUpTo' (-1) (not . visible) $ unArg trhs
(body, with) <- do
let n = size htel
addCtxTel htel $ checkWhere (size delta + n) wh $
-- for the body, we remove the implicits again
escapeContext n $
handleRHS aps (unArgs trhs) rhs0
escapeContext (size delta) $ checkWithFunction with
reportSDoc "tc.lhs.top" 10 $ escapeContext (size delta) $ vcat
[ text "Clause before translation:"
, nest 2 $ vcat
[ text "delta =" <+> prettyTCM delta
, text "perm =" <+> text (show perm)
, text "ps =" <+> text (show ps)
, text "body =" <+> text (show body)
, text "body =" <+> prettyTCM body
]
]
return $
Clause { clauseRange = getRange i
, clauseTel = killRange delta
, clausePerm = perm
, namedClausePats = ps
, clauseBody = body
, clauseType = Just trhs
}
src/full/Agda/TypeChecking/Rules/Def.hs:328:5:
Parse error in pattern: checkLeftHandSide
Possibly caused by a missing 'do'?
Trac metadata
Trac field | Value |
---|---|
Version | 7.10.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | high |
Resolution | Unresolved |
Component | Compiler (Parser) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |