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/-!
# Lenses
A `Lens` bundles a getter and an (updating) setter which can be used to elegantly describe a field of a structure.
-/
namespace Wayland
/-- A lens that points into a substructure of type `I` within a structure of type `O`. -/
structure Lens (O I : Type) where
/-- Get the substructure from the outer structure. -/
get : O → I
/-- Update the outer structure to contain the updated substructure. -/
set : I → O → O
def Lens.id (X : Type) : Lens X X where
get x := x
set x _ := x
def Lens.compose {X Y Z : Type} (l₁ : Lens X Y) (l₂ : Lens Y Z) : Lens X Z where
get := l₂.get ∘ l₁.get
set i o := l₁.set (l₂.set i (l₁.get o)) o
scoped infixr:80 " ≫ " => Lens.compose
/-- Helper for writing a `Lens` constructor for a named field of a structure. -/
macro "mk_lens " field:ident : term => do
let lvalNode := Lean.mkNode `Lean.Parser.Term.structInstLVal #[field.raw, Lean.mkNullNode #[]]
let lval : Lean.TSyntax `Lean.Parser.Term.structInstLVal := ⟨lvalNode⟩
`({ get := fun x => x.$field, set := fun i x => { x with $lval := i } })
end Wayland
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