"GHC.Integer can't throw exceptions" is wrong

The various integer packages, and anything else that might be loaded before the Prelude, goes through contortions to report errors, or else doesn't report them and crashes, because those packages are compiled before the Exception type is available.

However, there is a way to throw exceptions from code that only has access to ghc-prim. It relies on the fact that the RTS itself throws an exception in a certain circumstance: when atomically is called from within atomically.

This gives us the following:

{-# LANGUAGE MagicHash, UnboxedTuples, NoImplicitPrelude #-}
import GHC.Prim
import GHC.Magic

atomicLoop :: State# RealWorld -> (# State# RealWorld, a #)
atomicLoop s = atomically atomicLoop s

exception :: a
exception = runRW# (\s -> case atomicLoop s of
  (# _, a #) -> a)

I think that integer-simple and integer-gmp, and maybe the very earliest parts of base, are the only packages that would benefit from this circumlocution; however, having the error be a catchable exception rather than a straight-up crash has benefits.

Priority is low because I don't think there have been any bugs regarding GHC.Integer crashing; I just think it might make the code a bit more elegant.

Trac metadata

Trac field	Value
Version	8.2.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
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