show-please

Improved Show instances for exception and other types

Monthly Downloads: 12

Programming language: Haskell

License: BSD 3-clause "New" or "Revised" License

Tags: Debug    

README

Debugging Tools

Debug.Show

This module exports a type V(V), along with a collection of Show instances (initially for the types in Control.Exception) that actually display the values as a Haskell expression, so that they can be fully understood and used by the Read class.

> show (userError "foo")
"user error (foo)"
> show (V (userError "foo"))
"IOError {ioe_handle = Nothing, ioe_type = UserError, ioe_location = \"\", ioe_description = \"foo\", ioe_errno = Nothing, ioe_filename = Nothing}"

Some exception types have less detailed Show implementations, but at least they include the type name:

> Left (e :: SomeException) <- (try (error "abc"))
> putStrLn (show e)
abc
> putStrLn (show (V e))
toException (ErrorCall: abc :: ErrorCall)

Now you can see what the real type of a SomeException is:

> error "foo" `catch` (\(e :: SomeException) -> pure (show (V e))) >>= putStrLn
toException (ErrorCall "foo" :: ErrorCall)
> readProcess "false" [] "" `catch` (\(e :: SomeException) -> pure (show (V e))) >>= putStrLn
.toException (IOError {ioe_handle = Nothing, ioe_type = OtherError, ioe_location = "readCreateProcess: false (exit 1)", ioe_description = "", ioe_errno = Nothing, ioe_filename = Nothing} :: IOException)
> (forkIO (hClose stdout) >>= (putStrLn . show)) `catch` (\(e :: SomeException) -> putStrLn (show (V e)))
ghc: <stdout>: hFlush: illegal operation (handle is closed)