Implement primitive procedure application

src/primitives.rs

@@ -0,0 +1,58 @@
+use crate::parser::Datum;
+use crate::parser::Datum::*;
+use std::mem;
+
+pub fn add(mut args: Vec<Datum>) -> Datum {
+    let mut r = 0;
+    for i in 1..args.len() {
+        let datum = mem::take(&mut args[i]);
+        if let Number(n) = datum {
+            r += n;
+        } else {
+            panic!("ADD -- not-all-args-are-numbers {:?}", args)
+        }
+    }
+    Number(r)
+}
+
+pub fn mul(mut args: Vec<Datum>) -> Datum {
+    let mut r = 1;
+    for i in 1..args.len() {
+        let datum = mem::take(&mut args[i]);
+        if let Number(n) = datum {
+            r *= n;
+        } else {
+            panic!("MUL -- not-all-args-are-numbers {:?}", args)
+        }
+    }
+    Number(r)
+}
+
+pub fn sub(mut args: Vec<Datum>) -> Datum {
+    if args.len() == 2 {
+        let datum = mem::take(&mut args[1]);
+        if let Number(n) = datum {
+            return Number(-n);
+        } else {
+            panic!("SUB -- not-all-args-are-numbers {:?}", args)
+        }
+    }
+
+    let mut r;
+    let datum = mem::take(&mut args[1]);
+    if let Number(n) = datum {
+        r = n;
+    } else {
+        panic!("SUB -- not-all-args-are-numbers {:?}", args)
+    }
+
+    for i in 2..args.len() {
+        let datum = mem::take(&mut args[i]);
+        if let Number(n) = datum {
+            r -= n;
+        } else {
+            panic!("SUB -- not-all-args-are-numbers {:?}", args)
+        }
+    }
+    Number(r)
+}