@hackage / th-lock

Recent GHC versions with option -j<N> where N greater than 1 can build modules concurrently. Usually it is cool thing, because build time drops, but there is a drawback in form of problems with TH code maintaining a shared state.

I encountered such issue while running tests for trace-embrace package. Package TH code has a shared state (immutable configuration file loaded once), which is not supposed to change after loading, but tests require to cover all cases/branches and achieving this in the scope of a single cabal test-suit leads to inconsistencies.

Spreading tests among multiple test suits cause code duplication and looks less elegant.

Compilation of conflicting modules can be serialized with ensureSerialCompilationVerbose.

{-# LANGUAGE TemplateHaskell #-}
module ModuleA where

import Language.Haskell.TH.Lock

ensureSerialCompilationVerbose

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{-# LANGUAGE TemplateHaskell #-}
module ModuleB where

import Language.Haskell.TH.Lock

ensureSerialCompilationVerbose