Free dictionary variables in elaborated core expressions

Simon Peyton Jones simonpj at
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

zonkTopLExpr full_expr


From: ghc-devs <ghc-devs-bounces at> On Behalf Of Yiyun Liu
Sent: 03 February 2020 02:03
To: ghc-devs at
Cc: James Parker <jp at>
Subject: Free dictionary variables in elaborated core expressions

Hi ghc-devs,

About 10 days ago, I made a thread about defining a function called elaborateExpr which turns a string into a core expression<> 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<> 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?


- Yiyun
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