Create a new interactive_t helper class

The purpose of this class is much like Emacs' (interactive) form: it
allows a value expression function to declare exactly how many
arguments, and of what type, it intends to receive.  It then offers
type-safe access to theese arguments in a consistent manner.

An example value expression function definition in C++:

    value_t fn_foo(call_scope_t& scope) {
      // We expect a string, an integer, and an optional date
      interactive_t args(scope, "sl&d");

      std::cout << "String  = " << args.get<string>(0)
                << "Integer = " << args.get<long>(1) << std::endl;

      if (args.has(2)) // was a date provided?
        std::cout << "Date    = " << args.get<date_t>(2) << std::endl;

      return NULL_VALUE;
    }

There is also an in_context_t<T> template, which finds the context type
T in the current scope hierarchy.  The in_context_t then also acts as a
smart pointer to reference this context object, in addition to serving
the same duty as interactive_t.  This combination of intent is solely
for the sake of brevity.

    value_t fn_bar(call_scope_t& scope) {
      in_context_t<account_t> env(scope, "sl&d");
      std::cout << "Account name = " << env->fullname()
                << "String arg   = " << env.get<string>(0)
                << std::endl;
      return NULL_VALUE;
    }

As you can see here, 'env' acts as a smart pointer to the required
context, and an object to extract the typed arguments.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/precmd.cc b/src/precmd.cc
index c02be263..4b623904 100644
--- a/src/precmd.cc
+++ b/src/precmd.cc
@@ -30,6 +30,7 @@
  */
 
 #include "precmd.h"
+#include "interactive.h"
 #include "report.h"
 
 namespace ledger {