eio

IO with Exceptions tracked on the type-level.

Note: The package is considered to be used with the QualifiedDo feature, so hence the support only of GHC-9.0 and upper.

Usage example

Since this is a literate haskell file, we need to specify all our language extensions and imports up front.

{-# LANGUAGE QualifiedDo #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE DeriveAnyClass #-}

It is recomended to use eio library qualified, as it reimplements many standard functions.

import Control.Exception (Exception)
import EIO (EIO)

import qualified EIO

Let's also define our own exception to play with:

data MyErr = MyErr
    deriving stock (Show)
    deriving anyclass (Exception)

The main function of our module will look like this:

main :: IO ()
main = EIO.runEIO safeMain

Let's now write the safe main function that should not have any exceptions pushed to the actual main function, as the list of exceptions on type level should be empty.

This means, that if we throw the exception inside but don't handle it properly, it won't type check, as the list of exceptions in EIO will contain at least one element:

-- - Type error!
safeMainWrong :: EIO '[] ()
safeMainWrong = EIO.do
    EIO.throw MyErr

And the error will exactly point out to the fact that the empty list is expected, but got non-empty instead:

error:
    • Couldn't match type: '[MyErr]
                     with: '[]
      Expected: EIO '[] ()
        Actual: EIO '[MyErr] ()
    • In a stmt of a qualified 'do' block: EIO.throw MyErr
      In the expression: EIO.do EIO.throw MyErr
      In an equation for ‘safeMain’: safeMain = EIO.do EIO.throw MyErr
   |
xx |     EIO.throw MyErr
   |     ^^^^^^^^^^^^^^^

In order for it to type check, we need to handle each thrown exception properly, so that you have an empty list in the end:

safeMain :: EIO '[] ()
safeMain = EIO.do
    EIO.return ()
    EIO.throw MyErr `EIO.catch` (\MyErr -> EIO.return ())