Several changes, see commit msg

* clean up all tests
* bugfix for zero value functions, and test
* removed expand function, put in snippets
* added doc strings to Symbol type
* added doc strings to symbol declarations
* implemented display for Args type
* wrote a help function
* wrote docstrings for all builtins and config vars

src/sym.rs

@@ -19,6 +19,7 @@ use crate::eval::eval;
 use crate::segment::{Ctr, Seg, Type};
 use std::collections::HashMap;
 use std::rc::Rc;
+use std::fmt;
 
 #[derive(Clone)]
 pub struct SymTable(HashMap<String, Symbol>);
@@ -66,6 +67,7 @@ pub struct Symbol {
     // for internal control flow constructs
     // eval() will not eval the args
     pub conditional_branches: bool,
+    pub docs: String,
 }
 
 impl SymTable {
@@ -203,6 +205,23 @@ impl Args {
     }
 }
 
+impl fmt::Display for Args {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Args::None => write!(f, "none"),
+            Args::Infinite => write!(f, "infinite, untyped"),
+            Args::Lazy(n) => write!(f, "{} args of any type", n),
+            Args::Strict(s) => {
+                write!(f, "types: ")?;
+                for arg in s {
+                    write!(f, "{} ", arg)?
+                }
+                Ok(())
+            }
+        }
+    }
+}
+
 impl Symbol {
     /* call
      * routine is called by eval when a symbol is expanded
@@ -229,10 +248,7 @@ impl Symbol {
             evaluated_args = args;
         }
 
-        if let Err(msg) = self.args.validate_inputs(evaluated_args) {
-            return Err(format!("failure to call {}: {}", self.name, msg));
-        }
-
+        self.args.validate_inputs(evaluated_args)?;
         match &self.value {
             ValueType::VarForm(ref f) => Ok(Box::new(*f.clone())),
             ValueType::Internal(ref f) => Ok(Box::new(f(evaluated_args, syms)?)),
@@ -251,6 +267,7 @@ impl Symbol {
                             name: f.arg_syms[n].clone(),
                             value: ValueType::VarForm(Box::new(evaluated_args[n].clone())),
                             args: Args::None,
+                            docs: format!("local argument to {}", f.arg_syms[n].clone()),
                             conditional_branches: false,
                         },
                     ) {
@@ -265,7 +282,7 @@ impl Symbol {
                     if let Ctr::Seg(ref data) = *iterate.car {
                         match eval(data, syms) {
                             Ok(ctr) => result = ctr,
-                            Err(e) => return Err(e),
+                            Err(e) => return Err(format!("evaluation failure\n{}", e)),
                         }
                     } else {
                         let temp = Seg::from_mono(iterate.car.clone());
@@ -277,7 +294,7 @@ impl Symbol {
                                     result = ctr;
                                 }
                             }
-                            Err(e) => return Err(e),
+                            Err(e) => return Err(format!("evaluation failure\n{}", e)),
                         }
                     }