Querying Core for information about variables

[Adam Sandberg Eriksson]

I think some of what you want can be had from the Name type, and it seems you can get a Name from a Var with varName. See https://hackage.haskell.org/package/ghc-8.10.1/docs/Name.html#g:3 for some things that Names contain. For the type of a Var it seems you could use varType (https://hackage.haskell.org/package/ghc-8.10.1/docs/Var.html#v:varType).

I really recommend looking at the Haddocks, that's how I figured out what to do with Name's etc.

Adam Sandberg Eriksson


On Mon, 13 Jul 2020, at 19:24, Siddharth Bhat wrote:
> Hello,
> 
> I'm trying to understand how to query information about `Var`s from a
> 
> Core plugin. Consider the snippet of haskell:
> 
> ```
> {-# LANGUAGE MagicHash #-}
> import GHC.Prim
> fib :: Int# -> Int#
> fib i = case i of 0# -> i; 1# -> i; _ -> (fib i) +# (fib (i -# 1#))
> main :: IO (); main = let x = fib 10# in return ()
> ```
> 
> That compiles to the following (elided) GHC Core, dumped right after desugar:
> 
> ```
> Rec {
> fib [Occ=LoopBreaker] :: Int# -> Int#
> [LclId]
> fib
>  = ...
> end Rec }
> 
> Main.$trModule :: GHC.Types.Module
> [LclIdX]
> Main.$trModule
>  = GHC.Types.Module
>  (GHC.Types.TrNameS "main"#) (GHC.Types.TrNameS "Main"#)
> 
> -- RHS size: {terms: 7, types: 3, coercions: 0, joins: 0/0}
> main :: IO ()
> [LclIdX]
> main
>  = case fib 10# of { __DEFAULT ->
>  return @ IO GHC.Base.$fMonadIO @ () GHC.Tuple.()
>  }
> 
> -- RHS size: {terms: 2, types: 1, coercions: 0, joins: 0/0}
> :Main.main :: IO ()
> [LclIdX]
> :Main.main = GHC.TopHandler.runMainIO @ () main
> ```
> 
> I've been using `occNameString . getOccName` to manipulate names of `Var`s from the Core
> module. I'm rapidly finding this insufficient, and want more information
> about a variable. In particular, How to I figure out:
> 
> 1. When I see the Var with occurence name `fib`, that it belongs to module `Main`?
> 2. When I see the Var with name `main`, whether it is `Main.main` or `:Main.main`?
> 3. When I see the Var with name `+#`, that this is an inbuilt name? Similarly
>  for `-#` and `()`.
> 4. When I see the binder $trModule, that it is added by GHC and has type `GHC.Types.Module`?
> 5. In general, given a Var, how do I decide where it comes from, and whether it is
>  user-defined or something GHC defined ('wired-in' I believe is the term I am
>  looking for)?
> 6. When I see a `Var`, how do I learn its type?
> 7. In general, is there a page that tells me how to 'query' Core/`ModGuts` from within a core plugin?
> 
> Pointers on how to get this information is much appreciated. Also, pointers on 
> "learning how to learn" --- that is, how I could have figured this out on my own /
> RTFMing better are also very appreciated!
> 
> Thanks a lot,
> ~Siddharth
> 
> 
> -- 
> https://bollu.github.io/
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> 
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