Question about ambiguous predicates in pattern bindings

[Richard Eisenberg]

I've been a bit under water of late, so I haven't gotten to respond to this. But is this superseded by your later email? If not, I'm happy to take a stab at an answer.

Thanks,
Richard

> On Jan 15, 2022, at 2:09 PM, Benjamin Redelings <benjamin.redelings at gmail.com> wrote:
> 
> Hi,
> 
> 1. I'm reading "A Static semantics for Haskell" and trying to code it up.  I came across some odd behavior with pattern bindings, and I was wondering if someone could explain or point me in the right direction.
> 
> Suppose you have the declaration
> 
>     (x,y) = ('a',2)
> 
> My current code is yielding:
> 
>     x :: Num a => Char
> 
>     y :: Num a => a
> 
> However, I notice that ghci gives x the type Char, with no constraint, which is what I would expect.  It also gives y the type 'Num b => b', so I don't think it is defaulting a to Int here.
> 
> The weird results from my code stem from rule BIND-PRED in Figure 15 of https://homepages.inf.ed.ac.uk/wadler/papers/staticsemantics/static-semantics.ps
> 
>     E  |-  bind ~~> \dicts : theta -> monobinds in bind : (LIE_{encl}, theta => LVE)
> 
> Here theta = ( Num a ) and LVE = { x :: Char, y :: a }.  So, theta => LVE is
> 
>     { x :: Num a => Char, y :: Num a => a }
> 
> The obvious thing to do is avoid changing a type T to Num a => T if T does not contain a.  Also I'm not totally sure if that trick gets to the bottom of the issue.  However, the paper doesn't mention define theta => LVE that way.  Is there something else I should read on this?
> 
> 2. If we just chop out predicates which mention variables not in the type ( == ambiguous predicates?) then I'm not totally sure how to create code for this.
> 
> I would imagine that we have something like
> 
>     tup dn = ('a', fromInteger dn 2)
> 
>     x = case (tup dn) of (x,y) -> x
> 
>     y dn case (tup dn) of (x,y) -> y
> 
> In this case its not clear where to get the `dn` argument of `tup` from, in the definition of `x`.  Can we pass in `undefined`?  Should we do something else?
> 
> If anyone can shed light on this, I would be grateful :-)
> 
> -BenRI
> 
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