Show type of most recent expression in GHCi

Currently, any expression resulting in a value with no Show instance results in an error message. This is confusing for new users (especially if they don't yet understand typeclasses) and overly verbose for experienced users. Instead, the REPL should simply display the type of the expression, as if the user had used :t.

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Trac field	Value
Version	6.10.4
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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Thanks for the suggestion.

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Priority	normal → low

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Priority	low → lowest

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What should the output be in case :set +t was active?

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Trac field	Value
TypeOfFailure	- → OtherFailure

I'm a new contributor(preparing my first patch) and I'm working on this, currently my patch is this: https://github.com/osa1/ghc/commit/d542b418d913b4aba04e497fb63b70108b058f48

This is probably far from being finished, I'm looking for reviews/comments on this. I'm not very happy with the way it is implemented but for now this is only way I could think.

If someone point me a batter way to do I'll happily change my patch.

Here's an example output: https://gist.github.com/osa1/6201852

One thing I didn't like about this output is the it part, to me it's not user friendly enough. I think we need to tell user that his value isn't showable so only it's type is shown.

I'm a little confused by the output generated in your gist. Is the t option assumed now? You don't seem to set it, but you do unset it. And, I think the output should be more explicit about that it's changing behavior based on the lack of a Show instance. Having the output sometimes be a value and sometimes be a type is confusing!

In another direction, should we maybe promote the use of :force? This allows us to see the value of certain structures that don't have a Show instance.

Sorry for confusion, +t was enabled by default(specified in my .ghci).

Instead of completely removing the following error.

λ: id

<interactive>:2:1:
    No instance for (Show (a0 -> a0)) arising from a use of `print'
    Possible fix: add an instance declaration for (Show (a0 -> a0))
    In a stmt of an interactive GHCi command: print it

I think it would be saner and possibly better to just append the :t of it to the preceding error message.

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Trac field	Value
CC	→ leroux@fezrev.com

its worth pointing out that a beginner friendly ghci doesn't need to be baked into ghc, see http://hackage.haskell.org/package/ghci-ng for how to make your own ghci

there may be value in having a "beginners-ghci" that is self contained / a client of ghc apis

Taking out of 'patch' status as a bit of work/discussion is still needed.

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Trac field	Value
CC	leroux@fezrev.com → hvr, leroux@fezrev.com

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Moving to 7.10.1.

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Moving to 7.12.1 milestone; if you feel this is an error and should be addressed sooner, please move it back to the 7.10.1 milestone.

Moving to 7.12.1 milestone; if you feel this is an error and should be addressed sooner, please move it back to the 7.10.1 milestone.

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