WIP lambda

* typesystem extended
* docstrings and callback
* tests defined, not implemented

src/stl/decl.rs

@@ -244,3 +244,44 @@ pub fn env_callback(_ast: &Seg, syms: &mut SymTable) -> Result<Ctr, String> {
     }
     Ok(Ctr::None)
 }
+
+pub const LAMBDA_DOCSTRING: &str = "Takes two arguments of any type.
+No args are evaluated when lambda is called.
+Lambda makes sure the first argument is a list of symbols (or 'arguments') to the lambda function.
+The next arg is stored in a tree to evaluate on demand.
+
+Example: (lambda (x y) (add x y))
+This can then be evaluated like so:
+((lambda (x y) (add x y)) 1 2)
+which is functionally equivalent to:
+(add 1 2)";
+
+pub fn lambda_callback(
+    ast: &Seg,
+    _syms: &mut SymTable
+) -> Result<Ctr, String> {
+    let mut args = vec![];
+    if let Ctr::Seg(ref arg_head) = *ast.car {
+        if !arg_head.circuit(&mut |arg: &Ctr| -> bool {
+            if let Ctr::Symbol(ref s) = *arg {
+                args.push(s.clone());
+                true
+            } else {
+                false
+            }
+        }) {
+            Err("all elements of first argumnets must be symbols".to_string())
+        } else {
+            if let Ctr::Seg(ref eval_head) = *ast.cdr {
+                Ok(Ctr::Lambda(UserFn{
+                    ast: Box::new(eval_head.clone()),
+                    arg_syms: args,
+                }))
+            } else {
+                Err("not enough args".to_string())
+            }
+        }
+    } else {
+        Err("first argument should be a list of symbols".to_string())
+    }
+}