Problem:

This below will cause a compiler error, because the compiler thinks that you are trying to 
make a boolean a function, which is impossible. The compiler doesn't know you are 
trying to acutally launch the function since the function is a type.

program Project1;

{$mode objfpc}{$H+}

type
 TIsItTrue = function: boolean;
 
var
  MyBoolean: Boolean;
  IsItTrue: TIsItTrue;

begin
 MyBoolean:= IsItTrue; //<---COMPILER ERROR
 readln;
end.

Solution:

This syntax below will solve the problem and launch your function for you.

program Project1;

{$mode objfpc}{$H+}

type
 TIsItTrue = function: boolean;
 
var
  MyBoolean: Boolean;
  IsItTrue: TIsItTrue;

begin
 MyBoolean:= IsItTrue();  //note brackets
 readln;
end.