Re: Question about ambiguous predicates in pattern bindings

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 But is this superseded by your later email?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 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-semantic...

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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