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Use Error type for exceptions #1

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Merged
paf31 merged 3 commits into from
Aug 7, 2014
Merged

Use Error type for exceptions #1

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6789792
Use Error type for exceptions
paf31 Aug 7, 2014
49b52a9
Use opaque type for Error
paf31 Aug 7, 2014
5c442e1
Use Error type from Globals
paf31 Aug 7, 2014
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19 changes: 16 additions & 3 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -4,11 +4,24 @@

### Types

data Exception :: * -> !
data Error :: *

data Exception :: !


### Type Class Instances

instance showError :: Show Error


### Values

catchException :: forall e r a. (e -> Eff r a) -> Eff (err :: Exception e | r) a -> Eff r a
catchException :: forall a eff. (Error -> Eff eff a) -> Eff (err :: Exception | eff) a -> Eff eff a

error :: String -> Error

message :: Error -> String

showErrorImpl :: Error -> String

throwException :: forall a e r. e -> Eff (err :: Exception e | r) a
throwException :: forall a eff. Error -> Eff (err :: Exception | eff) a
32 changes: 28 additions & 4 deletions src/Control/Monad/Eff/Exception.purs
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Expand Up @@ -2,14 +2,34 @@ module Control.Monad.Eff.Exception where

import Control.Monad.Eff

foreign import data Exception :: * -> !
foreign import data Exception :: !

foreign import data Error :: *

instance showError :: Show Error where
show = showErrorImpl

foreign import showErrorImpl
"function showErrorImpl(err) {\
\ return err.stack ? err.stack : err.toString();\
\}" :: Error -> String

foreign import error
"function error(msg) {\
\ return new Error(msg);\
\};" :: String -> Error

foreign import message
"function message(e) {\
\ return e.message;\
\}" :: Error -> String

foreign import throwException
"function throwException(e) {\
\ return function() {\
\ throw e;\
\ };\
\}" :: forall a e r. e -> Eff (err :: Exception e | r) a
\}" :: forall a eff. Error -> Eff (err :: Exception | eff) a

foreign import catchException
"function catchException(c) {\
Expand All @@ -18,8 +38,12 @@ foreign import catchException
\ try {\
\ return t();\
\ } catch(e) {\
\ return c(e)();\
\ if (e instanceof Error) {\

@joneshf joneshf Sep 1, 2014

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What's the expected semantics here?

When would e not be an Error? If someone throws something that isn't in the Error hierarchy? Could we wrap that thing in an Error and handle it immediately, rather than throwing something we can't actually catch?

if (e instanceof Error) {
  return c(e)();
} else {
  return c(new Error(e))();
}

As it is now. This is a runtime exception we can't actually handle.

Unfortunately, it's also not a matter of wrapping the library throwing the non-error and calling it a day. This check will fail in NodeWebKit, since it uses two different contexts with different prototypes. So, what is thrown as an Error on the node side, will not be an instance of the window's Error because the prototypes are different. There's no way around this prototypal difference

I understand you shouldn't be concerned with what some random library does, but I think this happens for anything with a different context, like iframes, so, this might be more painful than first thought.

\ return c(e)();\
\ } else {\
\ throw e;\
\ }\
\ }\
\ };\
\ };\
\}" :: forall e r a. (e -> Eff r a) -> Eff (err :: Exception e | r) a -> Eff r a
\}" :: forall a eff. (Error -> Eff eff a) -> Eff (err :: Exception | eff) a -> Eff eff a