add documentation

src/nf/nf_loss.f90

@@ -1,8 +1,11 @@
 module nf_loss
 
-  !! This module will eventually provide a collection of loss functions and
-  !! their derivatives. For the time being it provides only the quadratic
-  !! function.
+  !! This module provides a collection of loss functions and their derivatives.
+  !! The implementation is based on an abstract loss derived type
+  !! which has the required eval and derivative methods.
+  !! An implementation of a new loss type thus requires writing a concrete
+  !! loss type that extends the abstract loss derived type, and that
+  !! implements concrete eval and derivative methods that accept vectors.
 
   implicit none
 
@@ -31,12 +34,14 @@ end function loss_derivative_interface
   end interface
 
   type, extends(loss_type) :: mse
+    !! Mean Square Error loss function
   contains
     procedure, nopass :: eval => mse_eval
     procedure, nopass :: derivative => mse_derivative
   end type mse
 
   type, extends(loss_type) :: quadratic
+    !! Quadratic loss function
   contains
     procedure, nopass :: eval => quadratic_eval
     procedure, nopass :: derivative => quadratic_derivative
@@ -73,7 +78,7 @@ end function mse_derivative
     pure module function quadratic_eval(true, predicted) result(res)
       !! Quadratic loss function:
       !!
-      !!   L  = (predicted - true)**2 / 2
+      !!   L  = sum((predicted - true)**2) / 2
       !!
       real, intent(in) :: true(:)
         !! True values, i.e. labels from training datasets