Utils: add type TransformF

Explain `Transform{,F}`
It allows easier reading.

Author: Anton-Latukha

ChangeLog.md

@@ -5,10 +5,14 @@
 
 * Additional:
 
+  * `Nix.Utils`:
+    * added type `TransformF`
+
   * `Nix.Eval`:
     * added fun
       * `evalContent`
 
+
 ### [(diff)](https://github.com/haskell-nix/hnix/compare/0.13.0.1...0.13.1#files_bucket) 0.13.1 (2021-05-22)
   * [(link)](https://github.com/haskell-nix/hnix/pull/936/files) `Nix.Parser`: `annotateLocation`: Fix source location preservation.
   * [(link)](https://github.com/haskell-nix/hnix/pull/934/files) Require Cabal dependency `relude` `>= 1.0`: since imports & Cabal file got cleaned-up & that clean-up depends on `relude` reimports introduced in aforementioned version.

src/Nix/Eval.hs

@@ -538,7 +538,7 @@ addSourcePositions f (v@(AnnE ann _) :: NExprLoc) =
 addStackFrames
   :: forall v e m a
    . (Scoped v m, Framed e m, Typeable v, Typeable m)
-  => Transform NExprLocF (m a)
+  => TransformF NExprLoc (m a)
 addStackFrames f v =
   do
     scopes <- currentScopes :: m (Scopes m v)

src/Nix/Utils.hs

@@ -34,21 +34,27 @@ traceM = const pass
 {-# inline traceM #-}
 #endif
 
-$(makeLensesBy (\n -> pure ("_" <> n)) ''Fix)
+$(makeLensesBy (\n -> pure $ "_" <> n) ''Fix)
 
 -- | > Hashmap Text -- type synonym
 type AttrSet = HashMap Text
 
--- | F-algebra defines how to reduce the fixed-point of a functor to a
---   value.
+-- | F-algebra defines how to reduce the fixed-point of a functor to a value.
 -- > type Alg f a = f a -> a
 type Alg f a = f a -> a
 
 -- | > type AlgM f m a = f a -> m a
 type AlgM f m a = f a -> m a
 
--- | "Transform" here means a modification of a catamorphism.
+-- | Do according transformation.
+--
+-- It is a transformation of a recursion scheme.
 type Transform f a = (Fix f -> a) -> Fix f -> a
+-- | Do according transformation.
+--
+-- It is a transformation of a recursion scheme.
+-- See @Transform@.
+type TransformF f a = (f -> a) -> f -> a
 
 loeb :: Functor f => f (f a -> a) -> f a
 loeb x = go