Find constraints over type

Simon Peyton Jones simonpj at
Fri Jan 17 08:53:43 UTC 2020

Starting from any point in the typechecked syntax tree, you can look outwards to find binding sites for enclosing constraints, and then use whatever method you like to decide if they are relevant. Binding sites are:

  *   ConPatOut; binds dictionaries from GADT matches
  *   AbsBinds: abs_ev_vars binds dictionaries from let-bindings


From: Alejandro Serrano Mena <trupill at>
Sent: 17 January 2020 08:26
To: Simon Peyton Jones <simonpj at>
Cc: GHC developers <ghc-devs at>
Subject: Re: Find constraints over type

My goal is to add type information on hover within `ghcide`. Right now, when you select a variable, we give back the type as reported in the corresponding Var. However, when this type involved a type variable, it misses a lot of important information about the typing context in which we are working. So the goal is to report back some of this typing context.

Going back to my original example:

f :: Eq a => [a] -> Bool
f xs = xs == []

It would be great if by hovering over the 'xs', one would get '[a] where Eq a', or some other representation of the known constraints.

Since this is intended to be a help for the programmer, it doesn't really matter whether we get "too many" constraints (for example, if we had "Ord a" it's OK to get "Eq a" too, since that's interesting constraint information).

Right now I am working with TypecheckModules most of the time.


El jue., 16 ene. 2020 a las 19:47, Simon Peyton Jones (<simonpj at<mailto:simonpj at>>) escribió:
There is definitely no pure way to get from ‘a’ to its constraints.

  *   It is far from clear what “its constraints” are.   Is (C a b) such a constraint?  C [a] b?  What about superclasses?
  *   Constraints vary depending on where  you are.  GADT matches can bring into scope extra constraints on existing type variables.

So as Ben says, to give a sensible response you’ll need to explain more about your goal


From: ghc-devs <ghc-devs-bounces at<mailto:ghc-devs-bounces at>> On Behalf Of Alejandro Serrano Mena
Sent: 16 January 2020 16:19
To: GHC developers <ghc-devs at<mailto:ghc-devs at>>
Subject: Find constraints over type

Dear GHC devs,
I am trying to figure out a way to obtain the constraints that hold over a type. Let me give you an example: suppose that I write the following function:

f :: Eq a => [a] -> Bool
f xs = xs == []

If I ask for the type of the Var ' xs', I get back '[a]'. This is correct, but I am missing the crucial information that '[a]' must be Eq.

Is there an easy way to get it? It seems that 'varType' doesn't give me enough information.

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