Control.Exception.evaluate

evaluate.hs

{-# LANGUAGE ScopedTypeVariables #-}

-- pure/return vs evaluate on try

data UserError = UserError String
  deriving Show
instance Exception UserError

>>> (try . pure . throw $ UserError "die") >>= either (\(e :: SomeException) -> putStrLn "exception captured") (\a -> seq a (putStrLn "no exception"))
*** Exception: foo

-- Why the exception isn't captured ?
--
-- Because pure doesn't evaluate the argument to whnf,
-- the exception is still not evaluated and try doesn't catch it.
--
-- Then when you call either it will match the Right thunk which contains
-- an exception and throws.

-- Use Control.Exception.evaluate instead.
-- Source: https://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Exception.html#v:evaluate

>>> (try . evaluate . throw $ UserError "die") >>= either (\(e :: SomeException) -> putStrLn "exception captured") (\a -> seq a (putStrLn "no exception"))
"exception captured"