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
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