Expr.Types: add notes to data {Params:ParamSet, NExprF:NSelect}

Some improvement ideas.

Grew to not like to put such in reports, since they get lost there and one can
implement them while at hand in the topic. (probably would do this during
breakage writing phase).

Author: Anton-Latukha

src/Nix/Expr/Types.hs

@@ -108,6 +108,8 @@ data Params r
   --
   -- > Param "x"                                  ~  x
   | ParamSet !(ParamSet r) !Bool !(Maybe VarName)
+  --  2021-05-15: NOTE: Seems like we should flip the ParamSet, so partial application kicks in for Bool?
+  --  2021-05-15: NOTE: '...' variadic property probably needs a Bool synonym.
   -- ^ Explicit parameters (argument must be a set). Might specify a name to
   -- bind to the set in the function body. The bool indicates whether it is
   -- variadic or not.
@@ -432,6 +434,8 @@ data NExprF r
   -- > NBinary NPlus x y                           ~  x + y
   -- > NBinary NApp  f x                           ~  f x
   | NSelect !r !(NAttrPath r) !(Maybe r)
+  --  2021-05-15: NOTE: Default value should be first argument to leverage partial application.
+  -- Cascading change diff is not that big.
   -- ^ Dot-reference into an attribute set, optionally providing an
   -- alternative if the key doesn't exist.
   --