diff --git a/CHANGELOG.md b/CHANGELOG.md
index 268e1ba5f..6106d741d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -71,12 +71,23 @@
 * Add an alias for `HVect` to `All id` in `Data.Vect.Quantifiers.All`. This is the
   approach to getting a heterogeneous Vect of elements that is generall preferred by
   the community vs. a standalone type as seen in `contrib`.
+* Add Data.List.HasLength from the compiler codebase slash contrib library but
+  adopt the type signature from the compiler codebase and some of the naming
+  from the contrib library. The type ended up being `HasLength n xs` rather than
+  `HasLength xs n`.
 
 #### System
 
 * Changes `getNProcessors` to return the number of online processors rather than
   the number of configured processors.
 
+
+#### Contrib
+* Remove Data.List.HasLength from contrib library but add it to the base library
+  with the type signature from the compiler codebase and some of the naming
+  from the contrib library. The type ended up being `HasLength n xs` rather than
+  `HasLength xs n`.
+
 ### Other Changes
 * The `data` subfolder of an installed or local dependency package is now automatically
   recognized as a "data" directory by Idris 2. See the
diff --git a/libs/contrib/Data/List/HasLength.idr b/libs/base/Data/List/HasLength.idr
similarity index 59%
rename from libs/contrib/Data/List/HasLength.idr
rename to libs/base/Data/List/HasLength.idr
index 1ef804efd..be6d647d2 100644
--- a/libs/contrib/Data/List/HasLength.idr
+++ b/libs/base/Data/List/HasLength.idr
@@ -12,8 +12,8 @@
 |||
 ||| We would write either of:
 ||| ```idris example
-||| f1 : (n : Nat) -> (0 _ : HasLength xs n) -> P xs
-||| f2 : (n : Subset n (HasLength xs)) -> P xs
+||| f1 : (n : Nat) -> (0 _ : HasLength n xs) -> P xs
+||| f2 : (n : Subset n (flip HasLength xs)) -> P xs
 ||| ```
 |||
 ||| See `sucR` for an example where the update to the runtime-relevant Nat is O(1)
@@ -23,6 +23,7 @@ module Data.List.HasLength
 
 import Data.DPair
 import Data.List
+import Data.Nat
 
 %default total
 
@@ -31,27 +32,45 @@ import Data.List
 
 ||| Ensure that the list's length is the provided natural number
 public export
-data HasLength : List a -> Nat -> Type where
-  Z : HasLength [] Z
-  S : HasLength xs n -> HasLength (x :: xs) (S n)
+data HasLength : Nat -> List a -> Type where
+  Z : HasLength Z []
+  S : HasLength n xs -> HasLength (S n) (x :: xs)
+
+||| This specification corresponds to the length function
+export
+hasLength : (xs : List a) -> HasLength (length xs) xs
+hasLength [] = Z
+hasLength (_ :: xs) = S (hasLength xs)
+
+export
+take : (n : Nat) -> (xs : Stream a) -> HasLength n (take n xs)
+take Z _ = Z
+take (S n) (x :: xs) = S (take n xs)
 
 ------------------------------------------------------------------------
 -- Properties
 
 ||| The length is unique
 export
-hasLengthUnique : HasLength xs m -> HasLength xs n -> m === n
+hasLengthUnique : HasLength m xs -> HasLength n xs -> m === n
 hasLengthUnique Z Z = Refl
 hasLengthUnique (S p) (S q) = cong S (hasLengthUnique p q)
 
-||| This specification corresponds to the length function
 export
-hasLength : (xs : List a) -> HasLength xs (length xs)
-hasLength [] = Z
-hasLength (_ :: xs) = S (hasLength xs)
+hasLengthAppend : HasLength m xs -> HasLength n ys -> HasLength (m + n) (xs ++ ys)
+hasLengthAppend Z ys = ys
+hasLengthAppend (S xs) ys = S (hasLengthAppend xs ys)
+
+hasLengthReverseOnto : HasLength m acc -> HasLength n xs -> HasLength (m + n) (reverseOnto acc xs)
+hasLengthReverseOnto p Z = rewrite plusZeroRightNeutral m in p
+hasLengthReverseOnto {n = S n} p (S q) = rewrite sym (plusSuccRightSucc m n) in hasLengthReverseOnto (S p) q
 
 export
-map : (f : a -> b) -> HasLength xs n -> HasLength (map f xs) n
+hasLengthReverse : HasLength m acc -> HasLength m (reverse acc)
+hasLengthReverse = hasLengthReverseOnto Z
+
+export
+map : (f : a -> b) -> HasLength n xs -> HasLength n (map f xs)
 map f Z = Z
 map f (S n) = S (map f n)
 
@@ -59,7 +78,7 @@ map f (S n) = S (map f n)
 ||| So performing this operation while carrying the list around would cost O(n)
 ||| but relying on n together with an erased HasLength proof instead is O(1)
 export
-sucR : HasLength xs n -> HasLength (snoc xs x) (S n)
+sucR : HasLength n xs -> HasLength (S n) (snoc xs x)
 sucR Z = S Z
 sucR (S n) = S (sucR n)
 
@@ -69,27 +88,27 @@ sucR (S n) = S (sucR n)
 namespace SubsetView
 
   ||| We provide this view as a convenient way to perform nested pattern-matching
-  ||| on values of type `Subset Nat (HasLength xs)`. Functions using this view will
+  ||| on values of type `Subset Nat (flip HasLength xs)`. Functions using this view will
   ||| be seen as terminating as long as the index list `xs` is left untouched.
   ||| See e.g. listTerminating below for such a function.
   public export
-  data View : (xs : List a) -> Subset Nat (HasLength xs) -> Type where
+  data View : (xs : List a) -> Subset Nat (flip HasLength xs) -> Type where
     Z : View [] (Element Z Z)
-    S : (p : Subset Nat (HasLength xs)) -> View (x :: xs) (Element (S (fst p)) (S (snd p)))
+    S : (p : Subset Nat (flip HasLength xs)) -> View (x :: xs) (Element (S (fst p)) (S (snd p)))
 
   ||| This auxiliary function gets around the limitation of the check ensuring that
   ||| we do not match on runtime-irrelevant data to produce runtime-relevant data.
-  viewZ : (0 p : HasLength xs Z) -> View xs (Element Z p)
+  viewZ : (0 p : HasLength Z xs) -> View xs (Element Z p)
   viewZ Z = Z
 
   ||| This auxiliary function gets around the limitation of the check ensuring that
   ||| we do not match on runtime-irrelevant data to produce runtime-relevant data.
-  viewS : (n : Nat) -> (0 p : HasLength xs (S n)) -> View xs (Element (S n) p)
+  viewS : (n : Nat) -> (0 p : HasLength (S n) xs) -> View xs (Element (S n) p)
   viewS n (S p) = S (Element n p)
 
   ||| Proof that the view covers all possible cases.
   export
-  view : (p : Subset Nat (HasLength xs)) -> View xs p
+  view : (p : Subset Nat (flip HasLength xs)) -> View xs p
   view (Element Z p) = viewZ p
   view (Element (S n) p) = viewS n p
 
@@ -102,13 +121,13 @@ namespace CurriedView
   ||| separately from the HasLength proof and the Subset view is not as useful anymore.
   ||| See e.g. natTerminating below for (a contrived example of) such a function.
   public export
-  data View : (xs : List a) -> (n : Nat) -> HasLength xs n -> Type where
+  data View : (xs : List a) -> (n : Nat) -> HasLength n xs -> Type where
     Z : View [] Z Z
-    S : (n : Nat) -> (0 p : HasLength xs n) -> View (x :: xs) (S n) (S p)
+    S : (n : Nat) -> (0 p : HasLength n xs) -> View (x :: xs) (S n) (S p)
 
   ||| Proof that the view covers all possible cases.
   export
-  view : (n : Nat) -> (0 p : HasLength xs n) -> View xs n p
+  view : (n : Nat) -> (0 p : HasLength n xs) -> View xs n p
   view Z Z = Z
   view (S n) (S p) = S n p
 
@@ -117,22 +136,24 @@ namespace CurriedView
 
 -- /!\ Do NOT re-export these examples
 
-listTerminating : (p : Subset Nat (HasLength xs)) -> HasLength (xs ++ [x]) (S (fst p))
-listTerminating p = case view p of
-  Z => S Z
-  S p => S (listTerminating p)
+listTerminating : (p : Subset Nat (flip HasLength xs)) -> HasLength (S (fst p)) (xs ++ [x])
+listTerminating p with (view p)
+  listTerminating (Element 0 Z) | Z = S Z
+  listTerminating (Element (S (fst y)) (S (snd y))) | (S y) = S (listTerminating y)
 
 data P : List Nat -> Type where
   PNil : P []
   PCon : P (map f xs) -> P (x :: xs)
 
 covering
-notListTerminating : (p : Subset Nat (HasLength xs)) -> P xs
-notListTerminating p = case view p of
-  Z => PNil
-  S p => PCon (notListTerminating {xs = map id (tail xs)} ({ snd $= map id } p))
+notListTerminating : (p : Subset Nat (flip HasLength xs)) -> P xs
+notListTerminating p with (view p)
+  notListTerminating (Element 0 Z) | Z = PNil
+  notListTerminating (Element (S (fst y)) (S (snd y))) | (S y) =
+    PCon (notListTerminating {xs = map id (tail xs)} ({ snd $= map id } y))
 
-natTerminating : (n : Nat) -> (0 p : HasLength xs n) -> P xs
+natTerminating : (n : Nat) -> (0 p : HasLength n xs ) -> P xs
 natTerminating n p = case view n p of
   Z => PNil
   S n p => PCon (natTerminating n (map id p))
+
diff --git a/libs/base/base.ipkg b/libs/base/base.ipkg
index 757075683..a46a0c126 100644
--- a/libs/base/base.ipkg
+++ b/libs/base/base.ipkg
@@ -58,6 +58,7 @@ modules = Control.App,
           Data.SnocList.Quantifiers,
           Data.SnocList.Operations,
           Data.List.Elem,
+          Data.List.HasLength,
           Data.List.Views,
           Data.List.Quantifiers,
           Data.List1,
diff --git a/libs/contrib/Data/List/AtIndex.idr b/libs/contrib/Data/List/AtIndex.idr
index db145ce13..ee8cf6c7b 100644
--- a/libs/contrib/Data/List/AtIndex.idr
+++ b/libs/contrib/Data/List/AtIndex.idr
@@ -93,24 +93,24 @@ weakenR Z = Z
 weakenR (S p) = S (weakenR p)
 
 export
-weakenL : (p : Subset Nat (HasLength ws)) -> AtIndex x xs n -> AtIndex x (ws ++ xs) (fst p + n)
-weakenL m p = case view m of
-  Z     => p
-  (S m) => S (weakenL m p)
+weakenL : (p : Subset Nat (flip HasLength ws)) -> AtIndex x xs n -> AtIndex x (ws ++ xs) (fst p + n)
+weakenL m p with (view m)
+  weakenL (Element 0 Z) p | Z = p
+  weakenL (Element (S (fst m)) (S (snd m))) p | (S m) = S (weakenL m p)
 
 export
-strengthenL : (p : Subset Nat (HasLength xs)) ->
+strengthenL : (p : Subset Nat (flip HasLength xs)) ->
               lt n (fst p) === True ->
               AtIndex x (xs ++ ys) n -> AtIndex x xs n
-strengthenL m lt idx = case view m of
-  S m => case idx of
-    Z => Z
-    S idx => S (strengthenL m lt idx)
+strengthenL m lt idx with (view m)
+  strengthenL (Element (S (fst m)) (S (snd m))) lt Z | (S m) = Z
+  strengthenL (Element (S (fst m)) (S (snd m))) lt (S k) | (S m) = S (strengthenL m lt k)
 
 export
-strengthenR : (p : Subset Nat (HasLength ws)) ->
+strengthenR : (p : Subset Nat (flip HasLength ws)) ->
               lte (fst p) n === True ->
               AtIndex x (ws ++ xs) n -> AtIndex x xs (minus n (fst p))
-strengthenR m lt idx = case view m of
-  Z => rewrite minusZeroRight n in idx
-  S m => case idx of S idx => strengthenR m lt idx
+strengthenR m lt idx with (view m)
+  strengthenR (Element 0 Z) lt idx | Z = rewrite minusZeroRight n in idx
+  strengthenR (Element (S (fst m)) (S (snd m))) lt (S k) | (S m) = strengthenR m lt k
+
diff --git a/libs/contrib/contrib.ipkg b/libs/contrib/contrib.ipkg
index 65d55079f..16fd18d75 100644
--- a/libs/contrib/contrib.ipkg
+++ b/libs/contrib/contrib.ipkg
@@ -46,7 +46,6 @@ modules = Control.ANSI,
           Data.List.Reverse,
           Data.List.Views.Extra,
           Data.List.Palindrome,
-          Data.List.HasLength,
           Data.List.AtIndex,
           Data.List.Alternating,
 
diff --git a/libs/papers/Data/OpenUnion.idr b/libs/papers/Data/OpenUnion.idr
index cc8423faf..d955626f6 100644
--- a/libs/papers/Data/OpenUnion.idr
+++ b/libs/papers/Data/OpenUnion.idr
@@ -58,7 +58,7 @@ prj = let (Element n p) = isMember t ts in prj' n p
 ||| By doing a bit of arithmetic we can figure out whether the union's value
 ||| came from the left or the right list used in the index.
 public export
-split : Subset Nat (HasLength ss) ->
+split : Subset Nat (flip HasLength ss) ->
         Union elt (ss ++ ts) -> Either (Union elt ss) (Union elt ts)
 split m (Element n p t) with (@@ lt n (fst m))
   split m (Element n p t) | (True ** lt)
@@ -92,5 +92,5 @@ weakenR (Element n p t) = Element n (weakenR p) t
 ||| the number of members introduced. Note that this number is the only
 ||| thing we need to keep around at runtime.
 public export
-weakenL : Subset Nat (HasLength ss) -> Union elt ts -> Union elt (ss ++ ts)
+weakenL : Subset Nat (flip HasLength ss) -> Union elt ts -> Union elt (ss ++ ts)
 weakenL m (Element n p t) = Element (fst m + n) (weakenL m p) t
diff --git a/src/Core/TT.idr b/src/Core/TT.idr
index 6c4e06149..8d78eba36 100644
--- a/src/Core/TT.idr
+++ b/src/Core/TT.idr
@@ -721,26 +721,31 @@ Show (Var ns) where
 
 namespace HasLength
 
+  -- TODO: delete in favor of base lib's List.HasLength once next version _after_ v0.6.0 ships.
   public export
   data HasLength : Nat -> List a -> Type where
     Z : HasLength Z []
     S : HasLength n as -> HasLength (S n) (a :: as)
 
+  -- TODO: Use List.HasLength.sucR from the base lib instead once next version _after_ v0.6.0 ships.
   export
   sucR : HasLength n xs -> HasLength (S n) (xs ++ [x])
   sucR Z     = S Z
   sucR (S n) = S (sucR n)
 
+  -- TODO: Use List.HasLength.hasLengthAppend from the base lib instead once next version _after_ v0.6.0 ships.
   export
   hlAppend : HasLength m xs -> HasLength n ys -> HasLength (m + n) (xs ++ ys)
   hlAppend Z ys = ys
   hlAppend (S xs) ys = S (hlAppend xs ys)
 
+  -- TODO: Use List.HasLength.hasLength from the base lib instead once next version _after_ v0.6.0 ships.
   export
   mkHasLength : (xs : List a) -> HasLength (length xs) xs
   mkHasLength [] = Z
   mkHasLength (_ :: xs) = S (mkHasLength xs)
 
+  -- TODO: Use List.HasLength.take from the base lib instead once next vresion _after_ v0.6.0 ships.
   export
   take : (n : Nat) -> (xs : Stream a) -> HasLength n (take n xs)
   take Z _ = Z
@@ -754,10 +759,12 @@ namespace HasLength
     succInjective : (0 l, r : Nat) -> S l = S r -> l = r
     succInjective _ _ Refl = Refl
 
+  -- TODO: Delete once version _after_ v0.6.0 ships.
   hlReverseOnto : HasLength m acc -> HasLength n xs -> HasLength (m + n) (reverseOnto acc xs)
   hlReverseOnto p Z = rewrite plusZeroRightNeutral m in p
   hlReverseOnto {n = S n} p (S q) = rewrite sym (plusSuccRightSucc m n) in hlReverseOnto (S p) q
 
+  -- TODO: Use List.HasLength.hasLengthReverse from the base lib instead once next version _after_ v0.6.0 ships.
   export
   hlReverse : HasLength m acc -> HasLength m (reverse acc)
   hlReverse = hlReverseOnto Z