Writing a simple Core evaluator, having trouble with name lookups

Christopher Done chrisdone at gmail.com
Tue Nov 27 17:59:38 UTC 2018

Hi all,

I'm attempting to make a simple evaluator for GHC core, but I'm not
clear on how to reliably looking up names. I'm compiling each of
ghc-prim, integer-simple and base with a patched version of GHC which
performs an extra output step with the Core AST to a file for each

Later, I load those files in. So for an input Haskell file like this:

    module Main (main,Foo(..)) where
    class Foo a where foo :: a -> Int
    instance Foo Int where foo x = x * x
    instance Foo Char where foo x = 99
    main = print (foo (123 :: Int))

I have an output set of bindings like this:


One thing that I immediately notice is that the names of things are
completely non-unique, especially in generated names. So here are two
implementations of the method "foo" for the class "Foo":

( Id {idStableName = "main:Main:$cfoo", idUnique = Unique
6989586621679010917}, ...) -- Int
( Id {idStableName = "main:Main:$cfoo", idUnique = Unique
6989586621679010923}, ...) -- Char

So e.g. the instance for "Foo Int" refers to the above method
implementation via its Unique (6989586621679010923):

( Id
    {idStableName = "main:Main:$fFooInt", idUnique = Unique
, CastE
          { idStableName = "main:Main:$cfoo"
          , idUnique = Unique 6989586621679010923 <---- HERE

At first, I thought I would use the Unique associated with every Name to
make a lookup. This is completely reliable within one GHC compilation,
but I've read in the docs that it's not stable across multiple
invocations? What does that mean for my case?

Another thing I notice is that type-class methods are not generated at
the core level. I have, for example, this method call that provides it
the instance dictionary,

                { idStableName = "main:Main:foo"
                , idUnique = Unique 8214565720323784707 <---- MISSING
          (TypE (Typ "Int")))
             { idStableName = "main:Main:$fFooInt"
             , idUnique = Unique 8214565720323784705

But the "main:Main:foo" (8214565720323784707) is not produced in the
CoreProgram, it seems. My compile step is very simple:

    compile ::
         GHC.GhcMonad m
      => GHC.ModSummary
      -> m [CoreSyn.Bind GHC.Var]
    compile modSummary = do
      parsedModule <- GHC.parseModule modSummary
      typecheckedModule <- GHC.typecheckModule parsedModule
      desugared <- GHC.desugarModule typecheckedModule
      let binds = GHC.mg_binds (GHC.dm_core_module desugared)
      pure binds

It simply gets the bindings and that's all from the ModGuts.

    mg_binds :: !CoreProgram

Two questions:

1) How do I recognize class methods when I see one, like the
   "main:Main:foo" above?

   Maybe this? isClassOpId_maybe :: Id -> Maybe Class

   Is an "op" what GHC calls type-class methods?

2) If I compile e.g. ghc-prim and that generates a binding Name with ID
   123, and then I compile base -- will the ID 123 be re-used by base
   for something else, or will any reference to 123 in the compiled
   Names for base refer ONLY to that one in ghc-prim? In other words,
   when GHC loads the iface for ghc-prim, does it generate a fresh set
   of names for everything in ghc-prim, or does it load them from file?

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