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@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'

_______________________________________________
Haskell-Cafe mailing list
To (un)subscribe, modify options or view archives go to:
http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
Only members subscribed via the mailman list are allowed to post.

 

--

brandon s allbery kf8nh

allbery.b@gmail.com

_______________________________________________
Haskell-Cafe mailing list
To (un)subscribe, modify options or view archives go to:
http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
Only members subscribed via the mailman list are allowed to post.