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7 January 2021 — by Cheng Shao, Richard Eisenberg
Haskell dark arts, part I: importing hidden values

You are a Haskeller debugging a large codebase. After hours of hopping around the source code of different modules, you notice some dirty and interesting code in one of your dependency’s Util or Internal module. You want to try calling a function there in your code, but hold on — the module (or the function) is hidden! Now you need to make your own fork, change the project build plan and do a lot of rebuilding. Some extra coffee break time is not bad, but what if we tell you this encapsulation can be broken, and you can import hidden functions with ease? Of course, this comes with some caveats, but no spoilers — read the rest of the post to find out how (and when).

Importing a hidden value with Template Haskell

Suppose we’d like to use the func top-level value defined in the Hidden module of the pkg package. We can’t simply import Hidden and use it if func is not exported or Hidden is not exposed. But don’t worry, with a single line of code in our own codebase, we can jailbreak the encapsulation:

myFunc = $(importHidden "pkg" "Hidden" "func")

myFunc can now be used just like the original func value. It doesn’t need to be defined as a top-level value; one can drop an importHidden splice anywhere. We only need to ensure the pkg package is a transitive dependency of the current package, enable the TemplateHaskell extension and import the module which implements importHidden.

The curious reader may check the Template Haskell API documentation and try to come up with their own importHidden implementation. It is well known that with Template Haskell, one can reify the information of datatypes and summon its hidden constructors, but summoning arbitrary hidden values is not directly supported. The next section reveals the secret.

Implementing the importHidden splice

Finding a package’s unit id

Let’s forget about importHidden for a minute and consider how to handwrite Haskell code to bring a hidden value into scope. Since we already know the package/module/value name, we can construct a Template Haskell Name that refers to the value, then use it to create the Exp that brings the value back. Time to give it a try in ghci:

Prelude> :set -XTemplateHaskell
Prelude> import Language.Haskell.TH.Syntax
Prelude Language.Haskell.TH.Syntax> myFunc = $(pure $ VarE $ Name (OccName "func") (NameG VarName (PkgName "pkg") (ModName "Hidden")))

<interactive>:3:12: error:
    • Failed to load interface for ‘Hidden’
      no unit id matching ‘pkg’ was found
    • In the expression: (pkg:Hidden.func)
      In an equation for ‘myFunc’: myFunc = (pkg:Hidden.func)

Oops, GHC complains that the pkg package can’t be found. The PkgName type in Template Haskell is a bit misleading here; GHC expects it to be the full unit ID of a package instead of the package name. What do unit IDs look like?

For packages shipped with GHC, they’re either the package name (e.g. base), or the package name followed by the version number (e.g. Cabal- However, unit IDs of third-party packages have a unique ABI hash suffix (e.g. aeson-, and the hash suffix differs if a package is built with different build plans. Thanks to this mechanism, most packages can be rebuilt multiple times and coexist in the same package database, a cabal build run will never fail due to version conflict with existing packages, and the so-called “cabal hell” becomes an ancient memory.

For importHidden to be useful, it needs to support third-party packages, therefore we need to find a way to query the exact unit ID given a package name via Template Haskell. Among the existing Template Haskell APIs, the closest thing to achieve this goal is reifyModule, which given a module name, returns its import list. So if Hidden appears in the current module’s import list, we can use reifyModule to get Hidden metadata which includes pkg’s unit ID. However, this approach has a significant restriction: it doesn’t work for hidden modules.

Abusing GHC API in Template Haskell

Recall that Template Haskell is usually run by a GHC process, so it’s possible to jailbreak the usual Template Haskell API and access the full GHC state when running a Template Haskell splice. The Q monad is defined as:

newtype Q a = Q { unQ :: forall m. Quasi m => m a }

This encodes a program that uses the Quasi class as its “instruction set”. In GHC, the typechecker monad TcM implements its Quasi instance which drives the actual Template Haskell logic. When running a splice, the type variable m is instantiated to TcM. If we can disguise a TcM a value as a Q a value, then we can access the full GHC session state inside TcM, which grants us access to the complete GHC API:

import DynFlags
import FastString
import Language.Haskell.TH.Syntax
import Module
import Packages
import TcRnMonad
import Unsafe.Coerce

unsafeRunTcM :: TcM a -> Q a
unsafeRunTcM m = unsafeCoerce (\_ -> m)

The implementation of unsafeRunTcM requires a bit of understanding about the dictionary-passing mechanism of type classes in GHC. The definition of Q can be interpreted as:

data QuasiDict m = QuasiDict {
  qNewName :: String -> m Name,

newtype Q a = Q { unQ :: forall m . QuasiDict m -> m a }

A QuasiDict m value is a dictionary which carries the implementation of Quasi methods in the m monad. A Q a value is a function which takes a QuasiDict m dictionary and calls the methods in it to construct a computation of type m a. When we instantiate m to a specific type constructor like TcM, GHC picks the corresponding dictionary and passes it to the function.

In our case, we know in advance that the Q a type is just a newtype of the Quasi m => m a computation which will be coerced to run in the TcM monad, therefore we can wrap a TcM a value in a lambda which discards its argument (which will be the Quasi instance dictionary for TcM) and coerce it to Q a. Another way to implement the coercion is:

unsafeRunTcM :: TcM a -> Q a
unsafeRunTcM m = Q (unsafeCoerce m)

The unsafeCoerce application must return a polymorphic value with the Quasi class constraint, and if we simply do unsafeRunTcM = unsafeCoerce, the resulting Q a value has the wrong function arity which leads to a segmentation fault at runtime.

Now that we can hook into GHC internal workings by running TcM a computations, it’s trivial to query the package state and find a package’s unit ID given its name. The rest of importHidden implementation follows:

qGetDynFlags :: Q DynFlags
qGetDynFlags = unsafeRunTcM getDynFlags

qLookupUnitId :: String -> Q UnitId
qLookupUnitId pkg_name = do
  dflags <- qGetDynFlags
  comp_id <- case lookupPackageName dflags $ PackageName $ fsLit pkg_name of
    Just comp_id -> pure comp_id
    _ -> fail $ "Package not found: " ++ pkg_name
  pure $ DefiniteUnitId $ DefUnitId $ componentIdToInstalledUnitId comp_id

qLookupPkgName :: String -> Q PkgName
qLookupPkgName pkg_name = do
  unit_id <- qLookupUnitId pkg_name
  pure $ PkgName $ unitIdString unit_id

importHidden :: String -> String -> String -> Q Exp
importHidden pkg_name mod_name val_name = do
  pkg_name' <- qLookupPkgName pkg_name
  pure $
    VarE $
        (OccName val_name)
        (NameG VarName pkg_name' (ModName mod_name))

Summarizing, our summoning ritual consists of:

  • Use unsafeCoerce to enable running a typechecker action in the Template Haskell Q monad.
  • Obtain the DynFlags of the current GHC session and query the package state to find a package’s full unit ID.
  • Construct a Name that refers to the hidden value and create the corresponding Exp.

With these hacks combined, now you can transcend the barriers of modules and packages!


Through a bit of knowledge about GHC internal workings, we practiced some Haskell dark arts and were able to summon hidden values. Before plugging this hack into a real-world codebase, let’s discuss the drawbacks of this approach.

If a top-level value isn’t exported, then the GHC inliner may choose to inline it at its call sites, therefore the interface file won’t contain its entry, and the summoning will fail at compile-time.

Given that we expect the splices to be run in the GHC process, it surely won’t work with an external interpreter or cross GHCs. On the other hand, for the particular use case of importHidden, we just need to query a package’s unit ID, so it should be fairly easy to patch GHC to support it when cross compiling: just add a method in the Quasi class, and support one more message variant in the external interpreter.

Running TcM actions in the Q monad is an interesting hack that doesn’t seem to have been used in the wild, and Richard Eisenberg has a nice video that introduces it. However, there’s a more principled way: GHC plugins, since they have full access to the GHC session state and can call arbitrary GHC API anyway.

Should you use importHidden? Most likely not, since patching the desired dependencies is always simpler and more robust. Nevertheless, it’s a fun exercise, and we hope this post serves as a peek into how GHC works under the hood :)

This article is licensed under a Creative Commons Attribution 4.0 International license.
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