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.

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Trac field	Value
Version	8.2.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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That's similar to what native metaprogramming would eventually allow you to do. Meanwhile, can't you already do this with Template Haskell (with a bit of more syntactic noise)?

Meanwhile, can't you already do this with Template Haskell (with a bit of more syntactic noise)?

I don't believe I can, as I need access to the type checker to see which things I can Show.

Thanks for the link to Native Metaprogramming, I'll have a read.

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Trac field	Value
Related	- → #14709 (closed)

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