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{-# LANGUAGE ScopedTypeVariables, DeriveDataTypeable #-}
module Main(main,f) where
import Data.List (isPrefixOf)
import Data.Dynamic
import Control.Exception as E
data Failure = Failure
deriving (Show, Typeable)
instance Exception Failure
test = (E.throw Failure >> return ())
`E.catch`
(\(x::Failure) -> return ())
main :: IO ()
main = print =<< test
f :: Bool -> Bool -> Bool
f True = error "urk"
-- f False = \y -> y
{-
Uderlying cause: we call
catch# thing handler
and expect that (thing state-token) will
- either diverge/throw an exception
- or return (# x,y #)
But it does neither: it returns a PAP, because
thing = \p q. blah
In particular, 'thing = lvl_sxo' is
lvl_sxc :: IO Any
lvl_sxc = error "urk"
lvl_sxo :: IO ()
= lvl_sxc >> return ()
-- inline (>>) --
= (\(eta::S#). case lvl_sxc |> g1 eta of ...) |> g2
where
g1 :: IO Any ~ S# -> (# S#, Any #)
g2 :: S# -> (# S#, () #) -> IO ()
-- case-of-bottomming function --
= (\ (eta::S#). lvl_sxc |> g1 |> ug3) |> g2
where
ug3(unsafe) :: S# -> (S#, Any) ~ (# S#, () #)
This is all fine. But it's crucial that lvl_sxc actually diverges.
Do not eta-expand it to
lvl_sxc :: IO Any
lvl_sxc = \eta. error "urk" |> ug4
where
ug4(unsafe) :: S# -> (# S#, Any #) ~ IO Any
In contrast, if we had
case x of
True -> \a -> 3
False -> error "urk"
we can, and must, eta-expand the error
-}
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