Add a plugin phase that allows users to modify HsSyn before the desugarer

I'm currently playing around with a little project that attempts to replicate py.test. For context about py.test, see https://docs.pytest.org/en/latest/example/reportingdemo.html#tbreportdemo, but essentially it's a Python library that provides an assert :: Bool -> IO () function, with the magic that if the assertion fails, it shows you some context about the Bool expression:

    def test_simple(self):
        def f():
            return 42
        def g():
            return 43

>       assert f() == g()
E       assert 42 == 43

For example, shows that f() evaluated to 42.

I'd like to do something like this for Haskell, and a GHC plugin seemed like the right place to do this. I first wrote a core-to-core plugin (https://github.com/ocharles/assert-explainer/blob/e6a669680cd214f23a4213e2f945f4468998fb1d/plugin/AssertExplainer.hs) which finds free variables and and attempts to Show them. This works, but I'd like to do more.

As an example of something I'd like to do, I would like to let my users write assert (and [x > 0 | x <- [-1, 0]). This is a failing assertion, and I would like to show a kind of evaluation trace, something like

and [x > 0 | x <- [-1, 0] = and [False, False] = False

To do this, I really need access to HsSyn, not CoreExpr. Template Haskell won't do, because I don't know what I can Show - I really need access to the type checker as well.

A plugin that fires right before the desugarer would be really nice for this kind of task.

Trac metadata

Trac field	Value
Version	8.2.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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