[Haskell-cafe] Can't get Type 3" to work

[Alexey Vagarenko]

Div was introduced in GHC 8.4, I believe.

пн, 3 дек. 2018 г. в 05:49, Amy de Buitléir <amy at nualeargais.ie>:

> Ah, thank you, that makes perfect sense. For anyone else who struggles
> with that example, you also need :set -XTypeOperators and import
> GHC.TypeLits.
>
> λ> :set -XDataKinds
> λ> :set -XTypeOperators
> λ> import GHC.TypeLits
> λ> :kind! (1 + 17) * 3
> (1 + 17) * 3 :: Nat
> = 54
>
> Unfortunately, I also get an error on the next line:
>
> λ> :kind! (Div 128 8) ^ 2
>
> <interactive>:1:2: error:
>     Not in scope: type constructor or class ‘Div’
>
> Div is defined in GHC.TypeLits, so I don't understand why that didn't work.
>
>
> On 2018-12-03 00:12, Brandon Allbery wrote:
>
> That looks like someone had a search-and-replace error.
>
> It used to be that the kind of normal values was *. Type level math made
> this problematic; by default, in recent ghc that kind is called Type
> instead. (See the StarIsType LANGUAGE pragma.)
>
> So someone did a search and replace to fix the kinds, and changed what
> should have been a type level multiplication by accident. Change it to a*.
>
> On Sun, Dec 2, 2018 at 7:09 PM Amy de Buitléir <amy at nualeargais.ie> wrote:
>
>> I'm reading "Thinking with Types" by Sandy Maguire, and there's a bit of
>> code on p. 27 that I don't understand and can't get to work. The "Type 3"
>> seems to have the effect of multiplying by 3.
>>
>> > :set -XDataKinds
>> >  :set -XTypeOperators
>> >  :kind! (1 + 17) Type 3
>>
>> (1 + 17) Type 3 :: Nat
>> = 54
>>
>> But it doesn't work for me. When I try this, I get...
>>
>> $ ghci --version
>> The Glorious Glasgow Haskell Compilation System, version 8.2.2
>>
>> $ ghci
>>
>> λ> :set -XDataKinds
>> λ>  :set -XTypeOperators
>> λ>  :kind! (1 + 17) Type 3
>>
>> <interactive>:1:2: error:
>>     Not in scope: type constructor or class '+'
>>
>> <interactive>:1:10: error:
>>     Not in scope: type constructor or class 'Type'
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>
>
>
> --
> brandon s allbery kf8nh
> allbery.b at gmail.com
>
>
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