"Cannot use function in a procedure call" compiler error - function

Question

Recursion towers of Hanoi program in ADA.
So far I think I have most of it down, my problem is being in my solve function.
I think I have the algorithm fine, but I am not sure how to implement it into the function, all examples I see of using this are using the function inside itself such as:
Example
My errors are:
hanoi.adb:23:09: cannot use function "solve" in a procedure call
hanoi.adb:27:09: cannot use function "solve" in a procedure call
hanoi.adb:59:15: missing ")"
Here is my code so far.
with ada.text_io, ada.command_line;
use ada.text_io, ada.command_line;
procedure hanoi is
Argument_Error : EXCEPTION;
max_disks, min_disks : integer := 3;
moves : integer := 0;
verbose_bool : boolean;
function solve (N: in integer; from, to, using: in character) return integer is
begin
if N = 1 then
if verbose_bool = true then
put("Move disk " & integer'image(N) & " from " & character'image(from) & " to " & character'image(to));
end if;
else
solve(N - 1, 'A', 'B', 'C');
if verbose_bool = true then
put("Move disk " & integer'image(N) & " from " & character'image(from) & " to " & character'image(to));
end if;
solve(N - 1, 'B', 'C', 'A');
end if;
moves := (2 ** min_disks) - 1;
return moves;
end solve;
begin
while min_disks /= max_disks loop
IF Argument_Count > 1 THEN
if Argument_Count = 1 then
min_disks := integer'value("Argument(1)");
elsif Argument_Count = 2 then
min_disks := integer'value("Argument(1)");
max_disks := integer'value("Argument(2)");
elsif Argument_Count = 3 then
min_disks := integer'value("Argument(1)");
max_disks := integer'value("Argument(2)");
if argument(3) = "v" or argument(3) = "V" then
verbose_bool := true; -- if argument is V or v it is true
end if;
END IF;
END IF;
IF Argument_Count > 3 THEN
RAISE argument_error;
END IF;
if (max_disks > 0) then
solve (N: integer; from, to, using : character);
END IF;
min_disks := min_disks + 1;
end loop;
EXCEPTION
WHEN Name_Error =>
Put_Line("Please re-enter your arguments, check to see if you entered integers and characters. Max of 3 arguments.");
WHEN OTHERS =>
Put_Line("Please try to not break the program again, thank you.");
end hanoi;

Answer 1

Functions return values, procedures do not, and you've defined Solve as a function.
Ada requires that you do something with a function's returned value, which you're not doing here. (You can't ignore the returned result as is done in other programming languages.)
As the error message states, your syntax is that of making a procedure call, i.e. invoking a procedure, but you've supplied the name of a function.
If the value being returned from a function is meaningful, then act on it in accordance with its purpose. If it is not providing any meaningful functionality, eliminate it and define Solve as a procedure.

Answer 2

As an aside, you may want to re-factor your display code into a nested subprogram. In the outline below, procedure Print can access the parameters of procedure Solve.
procedure Solve (N: in Integer; From, To, Using: in Character) is
procedure Print is
begin
if Verbose then
...
end if;
end Print;
begin
if N = 1 then
Print;
else
Solve (N - 1, 'A', 'B', 'C');
Print;
Solve (N - 1, 'B', 'C', 'A');
end if;
end Solve;

Answer 3

In addition to Marc's comment about the call to Solve's not being a proper Ada function reference, the syntax you have is that of a specification and not that of a invocation of Solve. You had it right in Solve's body just not in the initial invocation:
if (max_disks > 0) then
solve (N: integer; from, to, using : character);
END IF;