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lesson 9 - template haskell

Combining Template Haskell with indefinite packages can net you some unpleasant surprises.

In this lesson, we have the following libraries:

  • core An indefinite library with a module signature. It also defines some TH code.
  • intermediate Another indefinite library which inherits its signature from core. It also does some splicing.
  • intermediate-th A concrete library, which defines TH code equivalent to the one in core.
  • lib-impl A concrete implementation library.
  • lesson9 The main executable, which depends on both intermediate and lib-impl.

You can check file Intermediate.hs to see the splice. The splice code from comes from the concrete library intermediate-th:

-- this works, no problem
$(Intermediate.TH.makeIdFunc ''Core.A)

If we compile and run the lesson with

cabal run lesson9-template-haskell

Everything should work correctly.

Where things go wrong: splice code from as-yet indefinite libraries

Suppose that in Intermediate.hs, instead of using the splice code from intermediate-th, we used the splice code from core, which happens to be exactly identical:

-- $(Intermediate.TH.makeIdFunc ''Core.A)
$(Core.TH.makeIdFunc ''Core.A)

Alas, we get an obscure error:

cabal run lesson9-template-haskell
...
ghc: ^^ Could not load 'lesson9zmtemplatezmhaskellzm1zi0zi0zi0zminplacezmcorezmAfo5JIVDJVcCScFF1FkCSzz_CoreziTH_makeIdFunc_closure', dependency unresolved. See top entry above.
GHC.ByteCode.Linker.lookupCE
During interactive linking, GHCi couldn't find the following symbol:
  lesson9zmtemplatezmhaskellzm1zi0zi0zi0zminplacezmcorezmAfo5JIVDJVcCScFF1FkCSzz_CoreziTH_makeIdFunc_closure
This may be due to you not asking GHCi to load extra object files,
archives or DLLs needed by your current session.  Restart GHCi, specifying
the missing library using the -L/path/to/object/dir and -lmissinglibname
flags, or simply by naming the relevant files on the GHCi command line.
Alternatively, this link failure might indicate a bug in GHCi.
If you suspect the latter, please report this as a GHC bug:
  https://www.haskell.org/ghc/reportabug

What is happening here? Why does the splice code from core cause an error?

The reason is that splice code from as-yet indefinite libraries can't work.

Why? When we build an indefinite library like core, it gets typechecked, but not compiled down to object code. That can't be done, because the library still has "holes" in it, in the form of unfilled module signatures. But that means that other indefinite libraries like intermediate can't run the splice code when they themselves get typechecked, because the splice code doesn't exist yet.

Moving the Intermediate.TH module to intermediate won't work either, because intermediate itself is indefinite.

In a way, this is a more annoying version of the TH stage restriction.

What if...

We added a core dependency to lesson9 and tried to splice Core.TH.makeIdFunc in Main.hs itself? Should it work, or not? What do you think?

See also