Alejandro Serrano Mena <

[hidden email]> 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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