lsp-test: Add MonadThrow instance to Session

[thomasjm]

A MonadThrow instance is useful in the Session monad so that you can make assertions in various test frameworks.

This adds exceptions to the dependencies for lsp-test, but it's a pretty common dependency (already used in lsp).

[pepeiborra]

What kind of assertions do you have in mind? Can you share an example?

[thomasjm]

Pretty much all test frameworks I'm aware of throw exceptions to indicate failures. For example, HUnit:

https://hackage.haskell.org/package/HUnit-1.6.2.0/docs/Test-HUnit-Base.html#v:assertEqual

[pepeiborra]

I'm interested in understanding your use case. Would you be able to share an example?

[thomasjm]

I'm testing an LSP server. Here's an example test:

  it "does document highlight" $ doSession documentHighlightCode $ \filename -> do
    ident <- openDoc filename "haskell"
    let desired = [
          DocumentHighlight (Range (Position 0 0) (Position 0 3)) (Just HkWrite)
          , DocumentHighlight (Range (Position 1 9) (Position 1 12)) (Just HkRead)
          ]
    getHighlights ident (Position 0 1) >>= (`shouldBe` List desired)

The doSession command here runs some setup and ultimately calls runSessionWithConfig with the callback, so the body of this test runs in the Session monad. The shouldBe call is the part that needs MonadThrow to work.

[michaelpj]

Huh, what do we do in HLS currently? I thought we did something like this already? It does seem nice to be able to put the assertions in the session body, though.

[pepeiborra]

We use liftIO, because the hunit assertion combinators that we use are not lifted to MonadThrow. I can see how the lifted version enabled by MonadThrow is nicer, so this change seems Ok

[michaelpj]

Oh, if you wanted to be helpful it looks like there's some stuff in the lsp-test tests that could be cleaned up with this!