Warnings disappear after saving a buffer about dante HOT 11 CLOSED

This is likely the same issue that intero had to work around: if you load the same file twice in ghci, and it has warnings but not errors, ghci does not report the the warnings the second time it is loaded.

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Ok, I take it back - that change just made it so that the warnings don't disappear if I save the file immediately after some changes, which they did before... So I'm back to 'no idea where the problem is'. I guess I can work around it by not saving the file immediately after some changes, which I do by reflex now after some unpleasant experiences with crashing APL environments.

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@zarybnicky We subsequently changed the code for that, so be sure to check out how it works in the latest intero. It's unfortunate that it adds some complexity.

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As often, I would recommend bringing this issue to the attention of the GHCi maintainers.
Furthermore, the speed of the checker would be impacted by doing anything else than a simple reload. (And in general I hate increasing the complexity just to work around upstream issues.)

from dante.

The reliable fix is here: chrisdone-archive/intero#468

GHCi doesn't report warnings because you changed the file too rapidly, below <1 second duration. So in reporting whatever the last warnings were, if your users make a change too rapidly, then it'll report warnings for old code. I had this situation in Intero. The change in 62c3057 will report incorrect warnings sometimes.

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@chrisdone This does not seem to happen with my setup. What is the sequence of steps to reproduce the bug?

from dante.

It's GHCi behavior:

bash-3.2$ cat > input.txt
:set -Wall -fobject-code
:!echo "x = 'a'" > x.hs
:l x.hs
:!echo "y = 'a'" > x.hs
:l x.hs
:q
bash-3.2$ cat input.txt | ghci
GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
Prelude> Prelude> Prelude> [1 of 1] Compiling Main             ( x.hs, x.o )

x.hs:1:1: warning: [-Wmissing-signatures]
    Top-level binding with no type signature: x :: Char
Ok, modules loaded: Main (x.o).
Prelude Main> Prelude Describe Main> Ok, modules loaded: Main (x.o).
Prelude Main> Leaving GHCi.

Notice there is no warning about y despite the fact that we changed the file. In your commit, it will report that there's no top-level binding for y (because you're caching the old results).

You can also miss error messages due to this:

bash-3.2$ cat > input-no-errors.txt
:set -Wall -fobject-code
:!echo "x = 'a'" > x.hs
:l x.hs
:!echo "y = 'a' * k" > x.hs
:l x.hs
:q
bash-3.2$ cat input-no-errors.txt | ghci
GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
Prelude> Prelude> Prelude> [1 of 1] Compiling Main             ( x.hs, x.o )

x.hs:1:1: warning: [-Wmissing-signatures]
    Top-level binding with no type signature: x :: Char
Ok, modules loaded: Main (x.o).
Prelude Main> Prelude Describe Main> Ok, modules loaded: Main (x.o).
Prelude Main> Leaving GHCi.

There should be a type error or an out of scope error about k. There isn't because we changed the file too quickly. This happened to me a lot in Intero until I fixed it. We can put a delay before writing the file and then it works properly:

bash-3.2$ cat > input-delay.txt
:set -Wall -fobject-code
:!echo "x = 'a'" > x.hs
:l x.hs
:!sleep 1
:!echo "x = 'a' * k" > x.hs
:l x.hs
:q
bash-3.2$ cat input-delay.txt | ghci
GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
Prelude> Prelude> Prelude> [1 of 1] Compiling Main             ( x.hs, x.o )

x.hs:1:1: warning: [-Wmissing-signatures]
    Top-level binding with no type signature: x :: Char
Ok, modules loaded: Main (x.o).
Prelude Main> Prelude Main> Prelude Main> [1 of 1] Compiling Main             ( x.hs, x.o )

x.hs:1:11: error: Variable not in scope: k :: Char
Failed, modules loaded: none.
Prelude> Leaving GHCi.

For whatever reason in the GHC API, it ignores files that have changed in <1 second since the last modification.

Editing with flycheck turned on will trigger this a lot. If you just type check on a key binding or file save, you won't notice it that often.

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Thanks for the detailed information. I confirm that I am not observing this problem with my version of GHCi (8.0.2) on linux (nixos).

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It might be an OS X behavior then.

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