For Haskell questions about involving the `-XDataKinds` extension in GHC. With -XDataKinds, GHC automatically promotes every suitable datatype to be a kind, and its (value) constructors to be type constructors.
Questions tagged [data-kinds]
155 questions
7
votes
2 answers
How to create a "kind class" in Haskell, or ad-hoc polymorphism at the type-level using type families
I'm studying the type family features of Haskell, and type level computation.
It appears it's quite easy to get parametric polymorphism at the type-level using PolyKinds:
{-# LANGUAGE DataKinds, TypeFamilies, KindSignatures, GADTs, TypeOperators,…
CMCDragonkai
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7
votes
0 answers
Normalize type family instance within Template Haskell splice
I'm using the genifunctors package to generate a functor instance for a type whose definition involves type families.
The first module defines the data type itself:
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds…
vlopez
- 594
- 4
- 9
6
votes
0 answers
Syntax for using datatype indexed by Nat
The following code tries to refine Clash's Unsigned type family at index 4 into Digit:
import Clash.Prelude
{-@ type Digit = {v : Unsigned 4 | v <= 9 } @-}
type Digit = Unsigned 4
{-@ foo :: Digit -> Digit @-}
foo = id @Digit
This leads to the…
Cactus
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6
votes
2 answers
Using a promoted data constructor as a phantom parameter
In Maguire's Thinking with Types, p. 29, there's an example of how to use a promoted data constructor as a phantom parameter. Here's a module that I wrote based on the example in the book.
{-# LANGUAGE DataKinds #-}
module Main where
import …
mhwombat
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- 53
6
votes
1 answer
Type-level list of a single type level-tuple in Haskell
Using DataKinds and TypeOperators, I can make type level-tuples of types, and type-level lists of types, but I cannot nest them:
> :k '['(Int, Int), '(Int, Int)]
error: parse error on input ‘'’`
I can make a list of multiple tuples:
> :k…
Greg C
- 75
- 1
- 4
6
votes
3 answers
Sequence over heterogeneous list in Haskell
Consider following definition of a HList:
infixr 5 :>
data HList (types :: [*]) where
HNil :: HList '[]
(:>) :: a -> HList l -> HList (a:l)
And a type family Map to map over typelevel lists:
type family Map (f :: * -> *) (xs :: [*]) where
…
radrow
- 6,419
- 4
- 26
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6
votes
1 answer
Pattern match phantom type
I'm trying to implement a CurrencyQty type that acts like a number tagged at compile time:
data Currency = Usd | Eur | Gbp
data CurrencyQty (a :: Currency) = CurrencyQty Double deriving (Num)
And now I want to implement a generic conversion…
Eddie Wang
- 63
- 3
6
votes
2 answers
Understanding this definition of HList
I'm relatively new to Haskell, and I'm trying to understand one of the definitions of HList.
data instance HList '[] = HNil
newtype instance HList (x ': xs) = HCons1 (x, HList xs)
pattern HCons x xs = HCons1 (x, xs)
I have a couple specific…
lcmylin
- 2,552
- 2
- 19
- 31
6
votes
1 answer
How to specify the type for a heterogenous collection in a GADT formulated AST?
I'd like to make a typed AST for a dynamic language. At present, I'm stuck on handling collections. Here's a representative code sample:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE…
troutwine
- 3,721
- 3
- 28
- 62
6
votes
3 answers
DataKinds and type class instances
The following example is a boiled-down version of my real-life problem. It seems to be in some way similar to Retrieving information from DataKinds constrained existential types, but I could not quite get the answers I was seeking.
Suppose we have a…
Tobias Weck
- 269
- 1
- 8
6
votes
1 answer
Defining custom type families over the Nat kind
How does one define new computation over types of kind GHC.TypeLits.Nat? I am hoping to be able to define a type family
type family WIDTH (n :: Nat) :: Nat
such that WIDTH 0 ~ 0 and WIDTH (n+1) ~ log2 n
Cactus
- 27,075
- 9
- 69
- 149
6
votes
2 answers
Can I write a function using DataKinds that returns a value of type encoded by the parameter?
Let's say I have a Currency type:
data Currency = USD | EUR | YEN
and a Money type that stores an int, and is parameterized by a given Currency (Currency is promoted to a kind with the DataKinds extension).
data Money :: Currency -> * where
…
Ramith Jayatilleka
- 2,132
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6
votes
2 answers
Building values dynamically with GADTs using Data Kinds
Why is it harder to build values with datakinds, while it's relatively easy to pattern match with them?
{-# LANGUAGE KindSignatures
, GADTs
, DataKinds
, Rank2Types
#-}
data Nat = Zero | Succ Nat
data…
banx
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- 4
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6
votes
2 answers
How exactly do kind lists work?
I've been reading on vinyl recently, which uses weird "list of kinds" kinda types. After reading a bit on kinds and vinyl, I've gotten somewhat of an intuitive understanding of them, and I've been able to hack this together
{-# LANGUAGE DataKinds,
…
Cubic
- 14,902
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- 92
6
votes
1 answer
Type Inference with Reflection and DataKinds
I'm having problems getting GHC to infer a type in a place where it should be obvious. Below is a complete snippet demonstrating the problem.
{-# LANGUAGE DataKinds, ScopedTypeVariables, KindSignatures, TypeOperators, GADTs #-}
import…
crockeea
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