feat: add map and toArray

Author: fgdorais

Batteries/Data/DArray/Basic.lean

@@ -78,6 +78,14 @@ private unsafe def popImpl (a : DArray (n+1) α) : DArray n fun i => α i.castSu
 private unsafe def copyImpl (a : DArray n α) : DArray n α :=
   unsafeCast <| a.data.extract 0 n
 
+private unsafe def toArrayImpl (a : DArray n fun _ => α) : Array α :=
+  unsafeCast a
+
+@[specialize]
+private unsafe def mapImpl (f : {i : Fin n} → α i → β i) (a : DArray n α) : DArray n β :=
+  let f := fun i x => (unsafeCast (f (i:=i.cast lcProof) (unsafeCast x)) : NonScalar)
+  unsafeCast <| a.data.mapIdx f
+
 private unsafe def foldlMImpl [Monad m] (a : DArray n α) (f : β → {i : Fin n} → α i → m β)
     (init : β) : m β :=
   if n < USize.size then
@@ -185,6 +193,16 @@ protected def push (a : DArray n α) (v : β) :
 protected def pop (a : DArray (n+1) α) : DArray n fun i => α i.castSucc :=
   mk fun i => a.get i.castSucc
 
+/-- Cast a dependent array with constant types to an array. `O(1)` if exclusive else `O(n)`. -/
+@[implemented_by toArrayImpl]
+protected def toArray (a : DArray n fun _ => α) : Array α :=
+  .ofFn fun i => a.get i
+
+/-- Applies `f` to each element of a dependent array, returns the array of results. -/
+@[implemented_by mapImpl]
+protected def map (f : {i : Fin n} → α i → β i) (a : DArray n α) : DArray n β :=
+  mk fun i => f (a.get i)
+
 /--
 Folds a monadic function over a `DArray` from left to right:
 ```