Posted on 2017-03-13
by Oleg Grenrus

This question was recently asked on `lens`

issue tracker. The short answer, by Edward Kmett is

Mainly because we have functions, which perfectly characterize getters.

Let's explore that in more detail:

This post is a literate Haskell file, and as we work with `lens`

, we need to import them.

```
{-# LANGUAGE RankNTypes, TypeOperators #-}
module NoAGetter where
import Control.Lens
import Data.Type.Equality -- for a small proof
```

Keep in mind AGetter, if it existed, wouldn't work with views or any of the more general combinators other than view. So it'd be a thing that only existed so we could turn it into a function.

Suppose, there is `AGetter`

:

```
-- | When you see this as an argument to a function, it expects a Getter.
type AGetter s a = LensLike' (Retext a) s a
```

then we could put `AGetter`

s into a container. Similarly as we have a version of `(^.)`

, the `(^#) :: s -> ALens s t a b -> a`

that works on ALens, we'll new combinator, say `(^-) :: s -> AGetter s a -> b`

.

To put it differently: The `ALens`

is isomorphic to `Lens`

, but we cannot use it directly as `Lens`

:

```
Control.Lens> :t view (_1 :: ALens' (Int, Bool) Int)
<interactive>:... error:
• Couldn't match type ‘Control.Lens.Internal.Context.Pretext
(->) Int Int (Int, Bool)’
with ‘Const Int (Int, Bool)’
Expected type: Getting Int (Int, Bool) Int
Actual type: ALens' (Int, Bool) Int
• In the first argument of ‘view’, namely
‘(_1 :: ALens' (Int, Bool) Int)’
In the expression: view (_1 :: ALens' (Int, Bool) Int)
```

we need to clone the `ALens`

, or use specialized combinators:

```
Control.Lens> :t view (cloneLens (_1 :: ALens' (Int, Bool) Int))
view (cloneLens (_1 :: ALens' (Int, Bool) Int))
:: Control.Monad.Reader.Class.MonadReader (Int, Bool) m => m Int
```

The question is, what's `Retext`

? If you explore the `lens`

documentation, you'll notice that `ALens`

uses `Pretext`

, which is better `Context`

, which is isomorphic to `exists s. (s, Lens s t a b)`

. By analogy, we need something isomorphic to `exists s. (s, Getter s a)`

. `Getter s a`

is isomorphic to `s -> a`

, so

`exists s. (s, Getter s a) ~ exists s. (s, s -> a) ~ a`

As `Retext`

needs to be a `Functor`

, we'll just add a phantom type-variable. So at the end, the `Retext`

is an old friend of us:

`type Retext = Const`

The `Context`

has more type arguments, and we cannot define a `Sellable`

instance, but it would be `sell = Const`

. Using this observation we can define `cloneGetter`

and `(^-)`

, by adopting definitions of the `cloneLens`

and the `(^#)`

:

```
cloneGetter :: AGetter s a -> Getter s a
cloneGetter g = to $ \s -> getConst (g Const s)
infixl 8 ^-
(^-) :: s -> AGetter s a -> a
s ^- g = getConst (g Const s)
```

And we can use these definitions:

```
*Main> ('a', 'b') ^- _1
'a'
*Main> let getters = [_1, _2] :: [AGetter (Int, Int) Int]
*Main> map (\g -> (42, 7) ^- g) getters
[42,7]
*Main> map (\g -> (42, 7) ^. cloneGetter g) getters
[42,7]
```

If we inline the type-aliases of `AGetter`

, we'll get a `Getting a s a`

:

```
AGetter s a
= LensLike' (Const a) s a
= LensLike (Const a) s s a a
= (a -> Const a a) -> s -> Const a s
= Getting a s a
```

If you really want to put `Getter`

s into containers, simplest solution seems to use `Getting a s a`

: It's already in `Control.Lens`

.

```
type AGetter2 s a = Getting a s a
cloneGetter2 :: AGetter2 s a -> Getter s a
cloneGetter2 g = to $ \s -> s ^. g
```

One should note, tat the `AGetter`

and the `AGetter2`

are exactly the same:

```
proof :: AGetter s a :~: AGetter2 s a
proof = Refl
```

The corollary of above, is that need `(^-)`

, `(^.)`

works well:

```
*Main> let getters2 = [_1, _2] :: [AGetter2 (Int, Int) Int]
*Main> map (\g -> (42, 7) ^. g) getters2
[42,7]
```

because of the type of `(^.) :: s -> Getting a s a -> a`

.

Yet, we still need to clone the `AGetter`

when the viewed type is changed (similarly as with `ALens`

):

```
*Main> (42, True) ^. _1 . re _Show
"42"
*Main> (42, True) ^# (_1 :: ALens' (Int, Bool) Int) . re _Show
<interactive>:... error: ...
*Main> (42, True) ^. (_1 :: AGetter (Int, Bool) Int) . re _Show
<interactive>:... error:
*Main> (42, True) ^. cloneLens (_1 :: ALens' (Int, Bool) Int) . re _Show
"42"
*Main> (42, True) ^. cloneGetter (_1 :: AGetter (Int, Bool) Int) . re _Show
"42"
```

On the other hand, just using getter *functions* is even simpler, though then we "escape" the `lens`

eco-system:

```
type AGetter3 s a = s -> a
cloneGetter3 :: AGetter3 s a -> Getter s a
cloneGetter3 = to
```

To put `AGetter3`

s into container, we need explicitly `view`

them:

```
*Main> let getters3 = [view _1, view _2] :: [AGetter3 (Int, Int) Int]
*Main> map (\g -> (42, 7) ^. cloneGetter3 g) getters3
[42,7]
```

So, *why there is no AGetter?* In this post I tried to clarify, that having one doesn't add any(?) benefit over plain *getter functions*. Yet, you are free to play with the definitions here. Please tell me (e.g. via twitter), if you find some applications where `AGetter`

is more elegant then a plain old function.

You can run this file with

`stack --resolver=nightly-2017-03-01 ghci --ghci-options='-pgmL markdown-unlit' agetter.lhs`

fetch the source from https://gist.github.com/phadej/fb02aea7de32fba94bc3de33de6ee2e6

Site proudly generated by Hakyll