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Alejandro Serrano Mena

16 Jan 2020 16 Jan '20

11:19 a.m.

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. Regards, Alejandro

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Ben Gamari

16 Jan 16 Jan

1:06 p.m.

Alejandro Serrano Mena writes:

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.

Indeed `varType` of `xs` will only give you the type `a`. How to get back to the constraints that mention `xs` is a bit trick and will depend upon where in the compiler you are. As far as I can tell, getting constraints after desugaring will be quite difficult since they will have been lowered to dictionaries at that point. If you have access to hsSyn then you can of course easily compute the constraints explicitly provided by the signature for `xs` (assuming there is one). However, this will of course miss any inferred constraints. During typechecking it may be possible to compute some sort of constraint set by looking at the typechecking environment, although I'll admit this sounds quite dodgy. Can you provide more details on what you are trying to do? Cheers, - Ben

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Simon Peyton Jones

1:47 p.m.

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 Simon From: ghc-devs On Behalf Of Alejandro Serrano Mena Sent: 16 January 2020 16:19 To: GHC developers 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. Regards, Alejandro

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Alejandro Serrano Mena

17 Jan 17 Jan

3:25 a.m.

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. Regards, Alejandro El jue., 16 ene. 2020 a las 19:47, Simon Peyton Jones (< simonpj@microsoft.com>) 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

Simon

*From:* ghc-devs *On Behalf Of *Alejandro Serrano Mena *Sent:* 16 January 2020 16:19 *To:* GHC developers *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.

Regards,

Alejandro

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Simon Peyton Jones

3:53 a.m.

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 Simon From: Alejandro Serrano Mena Sent: 17 January 2020 08:26 To: Simon Peyton Jones Cc: GHC developers 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. Regards, Alejandro El jue., 16 ene. 2020 a las 19:47, Simon Peyton Jones (mailto:simonpj@microsoft.com>) 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 Simon From: ghc-devs mailto:ghc-devs-bounces@haskell.org> On Behalf Of Alejandro Serrano Mena Sent: 16 January 2020 16:19 To: GHC developers mailto:ghc-devs@haskell.org> 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. Regards, Alejandro

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Alejandro Serrano Mena
Ben Gamari
Simon Peyton Jones