Data.Error

Documentation

data Error

The canonical Error type.

It can be

• created from a human-readable error message (newError)
• more semantic context can be added to an existing Error (addContext)
• pretty-printed (prettyError)

newError :: Text -> Error

Create an ad-hoc Error from an error message.

addContext :: Text -> Error -> Error

Add a higher-level context to an Error.

For example, your code hits a “file not found” I/O exception. Instead of propagating it unseen, you catch it and annotate it with addContext, and describe why you wanted to open the file in the first place:

addContext "Trying to open config file"
  $ newError "file not found: ./foo"

This way, when a user see the error, they will understand better what happened:

"Trying to open config file: file not found: ./foo"

See prettyError.

prettyError :: Error -> Text

Pretty print the error.

It will print all context messages, starting with the outermost.

Example:

prettyError $ newError "file not found: ./foo"

==> "file not found: ./foo"

prettyError
  $ addContext "Trying to open config file"
    $ newError "file not found: ./foo"

==> "Trying to open config file: file not found: ./foo"

unwrapError :: Either Error p -> p

Return the value from a potentially failing computation.

Abort with the Errors message if it was a Left.

Panic: if Error

Example:

unwrapError $ Left (newError "oh no!")

==> *** Exception: oh no!

unwrapError $ Right 42

==> 42

expectError :: Text -> Either Error p -> p

Return the value from a potentially failing computation.

Abort with the error message if it was an error.

The text message is added to the Error as additional context before aborting.

Panic: if Error

Example:

exceptError "something bad happened" $ Left (newError "oh no!")

==> *** Exception: something bad happened: oh no!

exceptError $ Right 42

==> 42