PrettyError

Pretty runtime error messages

These functions can be used to raise clear, understandable error messages for when your program violates an invariant at runtime.

In particular, assertRight handles a common case where we "know" that a particular Either is going to be Right, and so we want to be able to unpack the value but also provide a high-quality error message if the value does somehow turn out to be a Left.

Documentation

assertRight

Arguments

:: Show a

=> Text

A message describing the assertion

-> Either a b

The Either to unpack

-> b

The unpacked Right value

Assert that value is Right. Throw error if it's Left.

The first argument is a message which can be used to explain why this error should not have happened.

Use fromRight if you don't want to provide a message.

>>> assertRight "Artificial example" (Right 5)
5

>>> assertRight "Artificial example" (Left "error message")
*** Exception: Artificial example: "error message"

fromRight :: Show a => Either a b -> b

Extract the value out of a Right, or throw an error from a Left made up of the pretty-printed Left value.

Most of the time you should use assertRight, since that allows you to provide a message, which will help to debug the problem.

>>> fromRight (Right 5)
5

>>> fromRight (Left 4)
*** Exception: 4

ppText :: Show a => a -> Text

Pretty print a variable to Text

Although not strictly related to error messages, this is a handy function to have around if you are using Text.Show.Pretty and OverloadedStrings.

prettyError :: Show a => a -> b

Raise an error with a pretty-printed value.

>>> prettyError ["foo","bar","baz"]
*** Exception: [ "foo" , "bar" , "baz" ]