runghc -fdefer-type-errors

[Simon Peyton-Jones]

Aha.  It is indeed true that

	ghc -fdefer-type-errors -w

does not suppress the warnings that arise from the type errors; indeed there is no current way to do so.  How to do that?

To be kosher there should really be a flag to switch off those warnings alone, perhaps
	-fno-warn-type-errors

So then -fwarn-type-errors is on by default, but is only relevant when -fdefer-type-errors is on.  Once -fdefer-type-errors is on, -fno-warn-type-errors and -fwarn-type-errors suppress or enable the warnings.  -w would then include -fno-warn-type-errors.

Is that a design everyone would like?  If so, woudl someone like to open a ticket, implement it, update the documentation, and send a patch?

many thanks

Simon

|  -----Original Message-----
|  From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-
|  bounces at haskell.org] On Behalf Of Richard Eisenberg
|  Sent: 11 March 2013 13:28
|  To: Kazu Yamamoto (山本和彦)
|  Cc: glasgow-haskell-users at haskell.org
|  Subject: Re: runghc -fdefer-type-errors
|  
|  When I ran this code (ghc 7.6.1), I did get the Hello, world! printout. That line
|  was sandwiched between the compile-time warning from the type error and the
|  run-time exception from the type error, but the output was there:
|  
|  09:24:28 ~/temp> runghc Scratch.hs
|  
|  Scratch.hs:5:12: Warning:
|      No instance for (Num String) arising from the literal `1'
|      Possible fix: add an instance declaration for (Num String)
|      In the first argument of `putStrLn', namely `1'
|      In a stmt of a 'do' block: putStrLn 1
|      In the expression:
|        do { putStrLn "Hello, world";
|             putStrLn 1 }
|  Hello, world
|  Scratch.hs: Scratch.hs:5:12:
|      No instance for (Num String) arising from the literal `1'
|      Possible fix: add an instance declaration for (Num String)
|      In the first argument of `putStrLn', namely `1'
|      In a stmt of a 'do' block: putStrLn 1
|      In the expression:
|        do { putStrLn "Hello, world";
|             putStrLn 1 }
|  (deferred type error)
|  
|  
|  It's easier to see with `runghc Scratch.hs 2> /dev/null` which prints only the
|  Hello, world! Oddly, passing flag "-w" doesn't suppress the warning, so I don't
|  think there's a way to turn it off.
|  
|  Richard
|  
|  On Mar 11, 2013, at 3:45 AM, Kazu Yamamoto (山本和彦) <kazu at iij.ad.jp> wrote:
|  
|  > Hello,
|  >
|  > Doesn't runghc support the -fdefer-type-errors option?
|  >
|  > Consider this code:
|  >
|  > ----
|  > module Main where
|  >
|  > main :: IO ()
|  > main = do
|  >    -- putStrLn は文字列を取る
|  >    putStrLn "Hello, world!"
|  >    putStrLn 1               -- 型エラー
|  > ----
|  >
|  > If I use runghc with -fdefer-type-errors, "Hello, world!" is not
|  > printed. Is this a bug?
|  >
|  > If this behavior is intended, I would like to change it. If GHC can
|  > run code like dynamically typed languages, it would be appealing to
|  > new Haskell programmers from their community.
|  >
|  > --Kazu
|  >
|  > _______________________________________________
|  > Glasgow-haskell-users mailing list
|  > Glasgow-haskell-users at haskell.org
|  > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
|  
|  
|  _______________________________________________
|  Glasgow-haskell-users mailing list
|  Glasgow-haskell-users at haskell.org
|  http://www.haskell.org/mailman/listinfo/glasgow-haskell-users