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

[amindfv at gmail.com]

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 <ollie at ocharles.org.uk> escribió:
> 
> "What's a call stack?"
> 
> (I don't know what Chris' target audience is though)
> 
> 
>> On Sat, 13 Feb 2016 5:18 pm Eric Seidel <eric at seidel.io> wrote:
>> Here's what the GHCi session should look like.
>> 
>> > $ 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
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