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

[Amy de Buitléir]

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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