Add null check to make_zero function
Protect against passing a NULL pointer to the function. Now it will
always never crash and it will return a NULL pointer if the parameter is
NULL.
Fix missing pattern in print_list
Add "catch-all" pattern in print_list function.
Add list data type example
Add list data type and data constructors inspired by Miran Lipovaca's
"Learn You a Haskell for Great Good".
To demonstrate the concept added length(), print_list() and map()
functions.
Add peano numbers implementation