I have something like this in my code:

```
data SomeKind = Kind1 | Kind2 deriving Eq
data SomeValue (t :: SomeKind) = SomeValue
someValue1 :: SomeValue Kind1
someValue1 = SomeValue
someValue2 :: SomeValue Kind2
someValue2 = SomeValue
```

I want to get the kind that's in the type level to the value level, perhaps something like:

```
valueKind :: SomeValue a -> SomeKind
```

that will:

```
valueKind someValue1 == Kind1
valueKind someValue2 == Kind2
```

Is it possible?

Answer:

`valueKind :: SomeValue a -> SomeKind`

is not possible. The `a`

type parameter isn't present runtime in any shape or form, so we can't branch on it.

The standard method for making use of types both at runtime and compile time is to make so-called singleton types. Singletons are indexed by the type-level version of the original type, and have the property that we can reveal the index by pattern matching on them:

```
data SSomeKind (i :: SomeKind) where
SKind1 :: SSomeKind Kind1
SKind2 :: SSomeKind Kind2
```

They're called singletons because for each type index there is only a single value. Similarly, for each value there is only a single choice of type index. This correspondence lets us use `SSomeKind`

as a runtime representation of `SomeKind`

.

```
valueKind' :: SSomeKind a -> SomeKind
valueKind' SKind1 = Kind1
valueKind' SKind2 = Kind2
```

Creating singleton definitions and associated lifting and lowering functions is a rather mechanical job. The `singletons`

library automates and streamlines it. In our case:

```
{-# LANGUAGE TemplateHaskell, DataKinds, GADTs, TypeFamilies, ScopedTypeVariables #-}
import Data.Singletons.TH
$(singletons [d| data SomeKind = Kind1 | Kind2 |])
-- generates a lot of stuff, including the SSomeKind definition.
-- works the same as our previously defined function
valueKind' :: SSomeKind a -> SomeKind
valueKind' = fromSing
-- we can also polymorphically get specific singleton values:
foo :: Sing Kind1
foo = sing -- now foo equals SKind1
```

There's a lot more in the library. See this page for a quick start and further references.

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