This is an inconsistency that was fixed in the revised Haskell98
report (Apr 2001). See
http://research.microsoft.com/~simonpj/haskell98-revised
In the bug list look for Page 90.
It looks as though Hugs hasn't quite caught up with this change.
Simon
| -----Original Message-----
| From: Brian Huffman [mailto:[EMAIL PROTECTED]]
| Sent: 22 March 2002 00:22
| To: [EMAIL PROTECTED]
| Subject: catch (fail "...")
|
|
| Hi,
|
| Here is a problem I came across recently, revealing a
| difference between ghci
| and Hugs:
|
| hugs> catch (error "not ok") (\e -> putStrLn "ok")
| Program error: not ok
| ghci> catch (error "not ok") (\e -> putStrLn "ok")
| *** Exception: not ok
|
| hugs> catch (ioError (userError "not ok")) (\e -> putStrLn "ok")
| ok
| ghci> catch (ioError (userError "not ok")) (\e -> putStrLn "ok")
| ok
|
| hugs> catch (fail "not ok") (\e -> putStrLn "ok")
| Program error: not ok
| ghci> catch (fail "not ok") (\e -> putStrLn "ok")
| ok
|
| As I expected, neither of them can catch "error", and both of
| them can catch
| "ioError". But only ghci can catch the "fail". It seems to me
| that ghc has
| made a convenient choice here, because if I need to raise an
| IO exception, it
| is easier to type "fail s" than "ioError (userError s)".
|
| However, the online Haskell report says that for IO, fail s =
| error s, which
| is what Hugs does.
| (http://www.haskell.org/onlinereport/standard-prelude.html#$iM
onad$$IO)
Does anyone know the reasoning behind the design choice made in the report,
or why the ghc implementation is different? I am mainly concerned about
portability issues: particularly, is it a good idea to use fail for raising
IO exceptions?
Thanks
- Brian Huffman
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