niteria authored Oct 30, 2015 and Ben Gamari committed Oct 30, 2015

When generating dictionary let binds in dsTcEvBinds we may
end up generating them in arbitrary order according to Unique order.

Consider:

```
let $dEq = GHC.Classes.$fEqInt in
let $$dNum = GHC.Num.$fNumInt in ...
```

vs

```
let $dNum = GHC.Num.$fNumInt in
let $dEq = GHC.Classes.$fEqInt in ...
```

The way this change fixes it is by using `UniqDFM` - a type of
deterministic finite maps of things keyed on `Unique`s. This way when
you pull out evidence variables corresponding to type-class dictionaries
they are in deterministic order.

Currently it's the order of insertion and the way it's implemented is by
tagging the values with the time of insertion.

Test Plan:
I've added a new test case to reproduce the issue.
./validate

Reviewers: ezyang, simonmar, austin, simonpj, bgamari

Reviewed By: simonmar, simonpj, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1396

GHC Trac Issues: #4012