Haskell equivalent of assert(0)

Question

I'm trying to figure out what is the best practice to write the Haskell equivalent of assert(0). I know that type safety dictates that an integer must be returned from scanList, however I wonder if there's a better way than what I wrote. Is there any way to avoid the arbitrary number 923 that is just stuck there?

module Main (main) where

import Control.Exception (assert)

l = [
    Left "1",
    Left "1",
    Right 1,
    Right 1,
    Left "9"]

scanList :: [ Either String Int ] -> Int
scanList [    ] = 0
scanList (x:xs) = case x of
    Right i -> i + scanList xs
    Left  s ->
        if read s < 8
        then read s + scanList xs
        else assert False $ 923

main = do
    print $ scanList l

Answer 1

From the documentation of assert:

If the first argument evaluates to True, then the result is the second argument. Otherwise an AssertionFailed exception is raised, containing a String with the source file and line number of the call to assert.

So instead of giving False as first argument you could actually check your if condition there:

scanList (x:xs) = case x of
    Right i -> i + scanList xs
    Left  s ->
        assert (read s < 8) (read s + scanList xs)