[Haskell-cafe] [] == []

[Yuji Yamamoto]

> And my main question is the second one :
For pedagogic purposes I want to get the error -- Whats the (family of)
related options?

This feature is called "type holes" introduced by GHC 7.8.1.
see https://wiki.haskell.org/GHC/Typed_holes in details.

2015-03-10 11:42 GMT+09:00 Rustom Mody <rustompmody at gmail.com>:

>
>
> On Tue, Mar 10, 2015 at 8:07 AM, Lyndon Maydwell <maydwell at gmail.com>
> wrote:
>
>> This looks like a great opportunity to try out the new holes
>> functionality:
>>
>> [Prelude] λ [] == _what
>>
>> <interactive>:3:7:
>>     Found hole ‘_what’ with type: [t0]
>>     Where: ‘t0’ is an ambiguous type variable
>>     Relevant bindings include it :: Bool (bound at <interactive>:3:1)
>>     In the second argument of ‘(==)’, namely ‘_what’
>>     In the expression: [] == _what
>>     In an equation for ‘it’: it = [] == _what
>>
>>
>> Looks like it defaults to [Bool]!
>>
>> Correct me if I'm wrong :)
>>
>
> Thanks Lyndon.
> But on my (debian testing) ghc I get:
>
> Prelude> [] == _what
>
> <interactive>:2:7: Not in scope: `_what'
>
> [ghc 7.6.3]
>
> And my main question is the second one :
> For pedagogic purposes I want to get the error -- Whats the (family of)
> related options?
>
>
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>
>


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