Improve error message when using rec/mdo keyword without RecursiveDo extention.
GHC shows unhelpful error message when using rec keyword and/or mdo keyword without RecursiveDo extention.
module NoRecursiveDo where
foo = do
rec str <- return "foo"
putStrLn str
$ ghc -c NoRecursiveDo.hs
NoRecursiveDo1.hs:4:5:
Parse error in pattern: rec
Possibly caused by a missing 'do'?
module NoRecursiveDo2 where
bar = mdo
str <- return "bar"
putStrLn str
$ ghc -c NoRecursiveDo2.hs
NoRecursiveDo2.hs:4:9: parse error on input `<-'
module NoRecursiveDo3 where
baz = mdo
putStrLn "baz"
$ ghc -c NoRecursiveDo3.hs
NoRecursiveDo3.hs:3:7:
Not in scope: `mdo'
Perhaps you meant `mod' (imported from Prelude)
I think that GHC should suggest to use RecursiveDo extention when using mdo/rec keyword.