Prisms are constructors

Posted on 2018-06-19 by Oleg Grenrus lens, notes

Prisms could be introduced as first-class pattern matching, but that is a one-sided view. I'd say they are generalised constructors, though maybe more often used for pattern matching than for actual construction.

The important property of constructors (and lawful prisms), is their injectivity. Though the usual prism laws don't state that directly, injectivity property can be deduced.

To quote lens-library documentation, the prisms laws are:

First, if I review a value with a Prism and then preview, I will get it back:

preview l (review l b) ≡ Just b

Second, if you can extract a value a using a Prism l from a value s, then the value s is completely described by l and a:

preview l s ≡ Just a  review l a ≡ s

In fact, the first law alone is enough to prove the injectivity of construction via Prism:

review l x ≡ review l y  x ≡ y

The proof is straight-forward:

review l x ≡ review l y
  -- x ≡ y -> f x ≡ f y
preview l (review l x) ≡ preview l (review l y)
  -- rewrite both sides with the first law
Just x ≡ Just y
  -- injectivity of Just
x ≡ y

We can use injectivity property as an additional tool in the equational reasoning toolbox. Or we can use it as a easy property to check to decide whether something is a lawful Prism. The check is easy as we only the review side of Prism. Many smart constructors, which for example normalise the input data, aren't lawful prisms.

An example using case-insensitive:

-- Bad!
_CI :: FoldCase s => Prism' (CI s) s
_CI = prism' ci (Just . foldedCase)

λ> review _CI "FOO" == review _CI "foo"

λ> "FOO" == "foo"

The first law is also violated:

λ> preview _CI (review _CI "FOO")
Just "foo"

#Pattern synonyms

We have to note, that as simply as you can construct lawless prisms, similarly simply you can construct "surprising" pattern synonyms

{-# LANGUAGE PatternSynonyms, ViewPatterns #-}
import Data.CaseInsensitive

pattern CI :: FoldCase a => a -> CI a
pattern CI a <- (foldedCase -> a) where CI a = mk a

λ> case CI "FOO" of CI s -> s

That pattern will be surprising to downstream users, precisely because it's not injective, therefore even it looks like a constructor, it's not.

For compiler writers it means, that expression case P x of P y -> z, where P is a pattern synonym, cannot be beta-reduced, but compiler needs to expand patterns first. (for a pattern synonym obeying prism laws, that would be safe to do).

#Dual: Surjectivity of Lens

Similarly, view through Lens is surjective:

forall a. exists s. view l s ≡ a

We can prove that using the first lens law, GetPut:

view l (set l a s) ≡ a

If there exists any value s', then the required s is set l a s, as then view l (set l a s) ≡ a, exactly the GetPut law. Otherwise, if there aren't any s' values, the reasoning isn't that interesting. This is classical argument, but hopefully I can hand wave through it.

-- There is a field for every type in the Void.
devoid :: Lens' Void Void a b
devoid = lens absurd (\v _ -> v)

#Appendix: Coq

The above is simple to formalise in Coq:

Variable Prism : Type -> Type -> Type.

Variable preview : forall {S A : Type} (p : Prism S A) (s : S), option A.
Variable review : forall {S A : Type} (p : Prism S A) (a : A), S.

Definition LawP1 (S A : Type) (p : Prism S A) : Type :=
 forall (a : A), preview p (review p a) = Some a.

Definition LawP2 (S A : Type) (p : Prism S A) : Type :=
  forall (s : S) (a : A),
    preview p s = Some a -> review p a = s.

Definition PrismInj (S A : Type) (p : Prism S A) : Type :=
  forall (x y : A),
    review p x = review p y -> x = y.

Lemma lemmaPrismInj (S A : Type) (p : Prism S A) :
  LawP1 S A p -> PrismInj S A p.
  unfold PrismInj. unfold LawP1.
  intros lawp1 x y H.
  assert (Some x = Some y).
  rewrite <- (lawp1 x).
  rewrite <- (lawp1 y).
  f_equal. exact H.
  injection H0. auto. Qed.

Variable Lens : Type -> Type -> Type.

Variable view : forall {S A : Type} (l : Lens S A) (s : S), A.
Variable set : forall {S A : Type} (l : Lens S A) (a : A) (s : S), S.

Definition LawL1 (S A : Type) (l : Lens S A) : Type :=
  forall (s : S) (a : A),   
    view l (set l a s) = a.

Definition LawL2 (S A : Type) (l : Lens S A) : Type :=
  forall (s : S), set l (view l s) s = s.

Definition LensSurj (S A : Type) (l : Lens S A) : Type :=
  forall (a : A), exists (s : S), view l s = a.

Lemma lemmaLensSurj (S A : Type) (l : Lens S A) (s : S) :
  LawL1 S A l -> LensSurj S A l.
  unfold LawL1. unfold LensSurj. intros lawl1 a.
  exists (set l a s).
  apply lawl1.


Thanks to Alp Mestanogullari for suggesting a section about PatternSynonyms.

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