Free dictionary variables in elaborated core expressions
Simon Peyton Jones
simonpj at microsoft.com
Mon Feb 3 09:18:33 UTC 2020
In your code (elabRnExpr) you have
_ <- perhaps_disable_default_warnings $ simplifyInteractive residual
You’ll notice that
simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind)
So you are discarding the “evidence bindings” returned by simplifyInteractive. Those are precisely the bindings of the dictionaries (dictionaries are a form of “evidence”) that you need. Don’t discard them.
ev_binds <- perhaps_disable_default_warnings $ simplifyInteractive residual
let full_expr = mkHsDictLet (EvBinds ev_binds) tc_expr
From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Yiyun Liu
Sent: 03 February 2020 02:03
To: ghc-devs at haskell.org
Cc: James Parker <jp at jamesparker.me>
Subject: Free dictionary variables in elaborated core expressions
About 10 days ago, I made a thread about defining a function called elaborateExpr which turns a string into a core expression<https://github.com/yiyunliu/ghc-elaboration-test/blob/8f362ad92dc6601b4cb7e4c76f0a42bc6b64480f/src/Main.hs#L55> within an interactive context. Now here's an unexpected behavior which I'm not sure how to deal with.
Given the expression:
(\x -> x + 1) :: Int -> Int
I expect to get something that looks like:
\ (x :: Int) -> + @ Int GHC.Num.$fNumInt x (GHC.Types.I# 1#)
where GHC.Num.$fNumInt is the exported dictionary.
What I actually get is something like this:
\ (x :: Int) -> + @ Int $dNum_someuniqueid x (GHC.Types.I# 1#)
where $dNum_someuniqueid is a free dictionary variable within the expression.
I was confused by the occurrence of the free variable $dNum at first, but after running the command: "ghc -ddump-ds-preopt somefile.hs" to dump the core bindings, I found that the dictionary variables like $dNum_ are actually local variables defined at the top-level.
My objective is to inline those top-level dictionary definitions into the core expression using let bindings, but it seems tricky since I'm doing everything within an interactive context. Calling getBindings<https://hackage.haskell.org/package/ghc-8.6.5/docs/GHC.html#v:getBindings> only gives me the expression I elaborated, but the dictionary is no where to be found.
Interestingly, when I turn on flags such as "DeferTypedHoles" or "DeferOutOfScopeVariables", all the dictionaries are defined locally in let bindings. However, I can't replicate that behavior even with the flags on in the interactive context. How do I find the dictionaries?
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