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The type hole feature is pretty cool!

I learned how to solve some of my type-related puzzlement by redefining stuff as "undefined" and using HDevTools to see what type gets inferred.

Also I can't wait to see what wild stuff people will come up with using the typechecker plugin API

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You've always been able to have type holes at the ghci prompt - wrap your expression in a lambda and see what's inferred as the first argument.

That's not to say the ability to do it in the middle of a program isn't huge - it is! - but just a technique I have occasionally found quite useful that's available to those stuck on older versions of ghc.

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