This expression was expected to have type unit but here has a type 'a list

Question

Here I have a function. In line List.append [0] free_list it is complaining that, This expression was expected to have type unit but here has a type 'a list, and it is not clear for me how does it infer that this statment must have an unit type.

let free_list=[]

let find_free_spaces  :List<int>=
   if  1=1                          // The condition is not important
   then  List.append [0] free_list
   free_list

Answer 1

In F#, if/then and if/then/else are expressions, not statements (if you're coming from C#, if x then y else z is like x ? y : z). If there's an else clause, then the type of that branch must match the type of the if branch so that the whole expression is well-typed. If there is no else, then the if's body must have type unit, since otherwise the result would just be disregarded (basically, if x then y is equivalent to if x then y else ()).