!! This is experimental work-in-progress !!

Welcome to error-context! This is a library providing context-aware error and exception handling for Haskell.

What problem does error-context attempt to solve?

Good error handling is hard. Sometimes it happens that when propagating errors some context is lost. Call traces sometimes help, but the current solutions in Haskell-land for accessing call traces are rather limited. Furthermore, sometimes call traces that at written by and for humans are more convenient to read.

The error-context library allows you to easily attach call traces ('error contexts') to errors, in particular to exceptions. Special catch-functions are provided for accessing these contexts.

How to use it?

Add an ErrorContextT layer to your monad transformer stack by adding runErrorContextT to the transformer unwrapping code.

The ErrorContextT transformer provides the context-enriching logic via special implementations of MonadThrow, MonadCatch and MonadIO.

Examples

See https://github.com/mtesseract/error-context/blob/master/test/Control/Error/Context/Test.hs.

What about "pure" exceptions?

The ErrorContextT transformer implements MonadThrow and MonadIO, therefore exceptions thrown by throwM and via liftIO are automatically context-enriched. But the story for exceptional values created via

throw :: Exception e => e -> a

are not context-enriched. But there is a workaround for this use-case:

ensureExceptionContext :: (MonadCatch m, MonadErrorContext m) => m a -> m a

This function provides context-aware enriching for any exceptions thrown within some monadic value, including those thrown by evaluating values created by throw.