Re: runghc -fdefer-type-errors

11 Mar 2013

      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@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
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Re: runghc -fdefer-type-errors

Richard Eisenberg