Re: [Haskell-cafe] New type of expressions containing (error ...) includes noisy implicit parameter

I don't have a copy of GHC 8 atm to test this with: is an expression like this now illegal? x :: Int x = undefined I.e. do you need to write: x :: HasCallStack => Int x = undefined Tom

El 13 feb 2016, a las 12:37, Oliver Charles escribió:

"What's a call stack?"

(I don't know what Chris' target audience is though)

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On Sat, 13 Feb 2016 5:18 pm Eric Seidel wrote: Here's what the GHCi session should look like.

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$ ghci GHCi, version 8.0.0.20160204: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/callen/.ghci Prelude> let myList = [1..5 :: Integer] Prelude> let myList' = myList ++ undefined Prelude> :t myList' myList' :: HasCallStack => [Integer]

If your readers are using :t they must already know about simple types like Integer, [], and, ->, so the new things are HasCallStack and =>. This is how I would explain them.

=> is just like -> except the compiler fills in the argument by itself. HasCallStack tells the compiler that the expression needs a call-stack because it might crash. So HasCallStack => [Integer] is a [Integer] that might crash and produce a stack-trace.

I think the call-stacks are much less scary and confusing than type-classes in general, which you kind of have to deal with as soon as you talk about arithmetic.

Eric _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

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