Add for statement and range function; New register_box_fn for functions returning Box<dyn Any> (which should not be boxed again); Register standard utility functions in library (e.g. print, string concat).

src/fn_register.rs

@@ -6,6 +6,9 @@ use crate::engine::{Engine, EvalAltResult};
 pub trait RegisterFn<FN, ARGS, RET> {
     fn register_fn(&mut self, name: &str, f: FN);
 }
+pub trait RegisterBoxFn<FN, ARGS> {
+    fn register_box_fn(&mut self, name: &str, f: FN);
+}
 
 pub struct Ref<A>(A);
 pub struct Mut<A>(A);
@@ -44,7 +47,37 @@ macro_rules! def_register {
 
                     // Call the user-supplied function using ($clone) to
                     // potentially clone the value, otherwise pass the reference.
-                    Ok(Box::new(f($(($clone)($par)),*)) as Box<dyn Any>)
+                    let r = f($(($clone)($par)),*);
+                    Ok(Box::new(r) as Box<dyn Any>)
+                };
+                self.register_fn_raw(name.to_owned(), Some(vec![$(TypeId::of::<$par>()),*]), Box::new(fun));
+            }
+        }
+
+        impl<$($par,)* FN> RegisterBoxFn<FN, ($($mark,)*)> for Engine
+        where
+            $($par: Any + Clone,)*
+            FN: Fn($($param),*) -> Box<dyn Any> + 'static
+        {
+            fn register_box_fn(&mut self, name: &str, f: FN) {
+                let fun = move |mut args: Vec<&mut dyn Any>| {
+                    // Check for length at the beginning to avoid
+                    // per-element bound checks.
+                    if args.len() != count_args!($($par)*) {
+                        return Err(EvalAltResult::ErrorFunctionArgMismatch);
+                    }
+
+                    #[allow(unused_variables, unused_mut)]
+                    let mut drain = args.drain(..);
+                    $(
+                    // Downcast every element, return in case of a type mismatch
+                    let $par = ((*drain.next().unwrap()).downcast_mut() as Option<&mut $par>)
+                        .ok_or(EvalAltResult::ErrorFunctionArgMismatch)?;
+                    )*
+
+                    // Call the user-supplied function using ($clone) to
+                    // potentially clone the value, otherwise pass the reference.
+                    Ok(f($(($clone)($par)),*))
                 };
                 self.register_fn_raw(name.to_owned(), Some(vec![$(TypeId::of::<$par>()),*]), Box::new(fun));
             }