Joachim Breitner's Homepage

I was wondering: What happens if I have a Haskell module with Template Haskell that embeds some information from the environment (time, environment variables). Will such a module be reliable recompiled? And what if it gets recompiled, but the source code produced by Template Haskell is actually unchanged (e.g., because the environment variable has not changed), will all depending modules be recompiled (which would be bad)?

Here is a quick experiment, using GHC-8.8:

/tmp/th-recom-test $ cat Foo.hs
{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -fforce-recomp #-}
module Foo where

import Language.Haskell.TH
import Language.Haskell.TH.Syntax
import System.Process

theMinute :: String
theMinute = $(runIO (readProcess "date" ["+%M"] "") >>= stringE)
[jojo@kirk:2] Mi, der 01.07.2020 um 17:18 Uhr ☺
/tmp/th-recom-test $ cat Main.hs
import Foo
main = putStrLn theMinute

Note that I had to set {-# OPTIONS_GHC -fforce-recomp #-} – by default, GHC will not recompile a module, even if it uses Template Haskell and runIO. If you are reading from a file you can use addDependentFile to tell the compiler about that depenency, but that does not help with reading from the environment.

So here is the test, and we get the desired behaviour: The Foo module is recompiled every time, but unless the minute has changed (see my prompt), Main is not recomipled:

/tmp/th-recom-test $ ghc --make -O2 Main.hs -o test
[1 of 2] Compiling Foo              ( Foo.hs, Foo.o )
[2 of 2] Compiling Main             ( Main.hs, Main.o )
Linking test ...
[jojo@kirk:2] Mi, der 01.07.2020 um 17:20 Uhr ☺
/tmp/th-recom-test $ ghc --make -O2 Main.hs -o test
[1 of 2] Compiling Foo              ( Foo.hs, Foo.o )
Linking test ...
[jojo@kirk:2] Mi, der 01.07.2020 um 17:20 Uhr ☺
/tmp/th-recom-test $ ghc --make -O2 Main.hs -o test
[1 of 2] Compiling Foo              ( Foo.hs, Foo.o )
[2 of 2] Compiling Main             ( Main.hs, Main.o ) [Foo changed]
Linking test ...

So all well!

Update: It seems that while this works with ghc --make, the -fforce-recomp does not cause cabal build to rebuild the module. That’s unfortunate.