Add more specific warning for error on no import, clarify docs.

doc/chibi.scrbl

@@ -136,6 +136,8 @@ documentation system described in
 build this manual.  \ccode{chibi-ffi} is a tool to build wrappers for
 C libraries, described in the FFI section below.
 
+See the examples directory for some sample programs.
+
 \section{Default Language}
 
 \subsection{Scheme Standard}
@@ -231,6 +233,15 @@ These forms perform basic selection and renaming of individual
 identifiers from the given module. They may be composed to perform
 combined selection and renaming.
 
+Note while the repl provides default bindings as a convenience,
+programs have strict semantics as in R7RS and must start with at least
+one import, e.g.
+
+\schemeblock{
+(import (scheme base))
+(write-string "Hello world!\n")
+}
+
 Some modules can be statically included in the initial configuration,
 and even more may be included in image files, however in general
 modules are searched for in a module load path.  The definition of the

eval.c

@@ -45,7 +45,9 @@ void sexp_warn (sexp ctx, const char *msg, sexp x) {
   if (sexp_oportp(out)) {
     sexp_write_string(ctx, strictp ? "ERROR: " : "WARNING: ", out);
     sexp_write_string(ctx, msg, out);
-    sexp_write(ctx, x, out);
+    if (x != SEXP_UNDEF) {
+      sexp_write(ctx, x, out);
+    }
     sexp_write_char(ctx, '\n', out);
     if (strictp) sexp_stack_trace(ctx, out);
   }
@@ -1100,8 +1102,13 @@ static sexp analyze (sexp ctx, sexp object, int depth, int defok) {
     } else if (sexp_idp(sexp_car(x))) {
       if (! cell) {
         res = analyze_app(ctx, x, depth);
-        if (sexp_exceptionp(res))
+        if (sexp_exceptionp(res)) {
           sexp_warn(ctx, "exception inside undefined operator: ", sexp_car(x));
+          /* the common case of no imports */
+          if (!sexp_env_parent(sexp_context_env(ctx))) {
+            sexp_warn(ctx, "did you forget to import a language? e.g. (import (scheme base))", SEXP_UNDEF);
+          }
+        }
       } else {
         op = sexp_cdr(cell);
         if (sexp_corep(op)) {

examples/hello.scm

@@ -0,0 +1,3 @@
+(import (scheme base))
+
+(write-string "Hello world!\n")