#1200: ghci scripts ending in printf lines fail with Exception:
Prelude.undefined
----------------------------+-----------------------------------------------
Reporter: dons | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.8
Component: libraries/base | Version: 6.6
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: Unknown
Os: Unknown |
----------------------------+-----------------------------------------------
Changes (by igloo):
* component: GHCi => libraries/base
* milestone: => 6.8
Comment:
The problem is that `main :: IO a`, and ghci now tries to print out the
value resulting from running an IO action (unless that value has type (),
which is why annotations fix it). runhaskell probably shouldn't share this
behaviour in my opinion.
printf has things like
{{{
instance PrintfType (IO a) where
spr fmt args = do
putStr (uprintf fmt (reverse args))
return undefined
}}}
so ghci tries to print out this undefined value.
We could change the instance to `IO ()` so that we could remove the last
line, but that's probably not portable enough. Making it
{{{
class Unit a where unit :: a
instance Unit () where unit = ()
instance Unit a => PrintfType (IO a) where
...
return unit
}}}
would work, but would give some possibly confusing type errors when a type
`IO a` is infered.
Or we could just change the `undefined`s to `error`s with an explanatory
message.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1200>
GHC <http://www.haskell.org/ghc/>
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