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forbid modifying constants

--- a/src/builtins.c

+++ b/src/builtins.c

@@ -175,8 +175,9 @@

 {

 	argcount(nargs, 2);

 	sl_sym *sym = tosym(args[0]);

-	if(!isconst(sym))

-		sym->binding = args[1];

+	if(sl_unlikely(isconst(sym)))

+		const_error(sym);

+	sym->binding = args[1];

 	return args[1];

 }

@@ -184,8 +185,9 @@

 {

 	argcount(nargs, 1);

 	sl_sym *sym = tosym(args[0]);

-	if(!isconst(sym))

-		sym->binding = UNBOUND;

+	if(sl_unlikely(isconst(sym)))

+		const_error(sym);

+	sym->binding = UNBOUND;

 	return sl_void;

 }

--- a/src/sl.c

+++ b/src/sl.c

@@ -18,7 +18,7 @@

 sl_v sl_booleansym, sl_nullsym, sl_evalsym, sl_fnsym, sl_trimsym;

 sl_v sl_nulsym, sl_alarmsym, sl_backspacesym, sl_tabsym, sl_linefeedsym, sl_newlinesym;

 sl_v sl_vtabsym, sl_pagesym, sl_returnsym, sl_escsym, sl_spacesym, sl_deletesym;

-sl_v sl_errio, sl_errparse, sl_errtype, sl_errarg, sl_errmem;

+sl_v sl_errio, sl_errparse, sl_errtype, sl_errarg, sl_errmem, sl_errconst;

 sl_v sl_errdiv0, sl_errbounds, sl_err, sl_errkey, sl_errunbound, sl_erroom;

 sl_v sl_emptyvec, sl_emptystr;

@@ -164,6 +164,17 @@

 }

 _Noreturn void

+const_error(const sl_sym *sym)

+{

+	lerrorf(

+		sl_errconst,

+		"modifying a %s is not permitted: %s",

+		iskeyword(sym) ? "keyword" : "constant",

+		sym->name

+	);

+}

+

+_Noreturn void

 unbound_error(sl_v sym)

 {

 	sl_raise(mk_listn(2, sl_errunbound, sym));

@@ -1253,6 +1264,7 @@

 	sl_errunbound = mk_csym("unbound-error");

 	sl_errkey = mk_csym("key-error");

 	sl_errmem = mk_csym("memory-error");

+	sl_errconst = mk_csym("const-error");

 	sl_errbounds = mk_csym("bounds-error");

 	sl_errdiv0 = mk_csym("divide-error");

 	sl_err = mk_csym("error");

--- a/src/sl.h

+++ b/src/sl.h

@@ -263,6 +263,7 @@

 _Noreturn void sl_raise(sl_v e);

 _Noreturn void type_error(const char *expected, sl_v got);

 _Noreturn void bounds_error(sl_v arr, sl_v ind);

+_Noreturn void const_error(const sl_sym *sym);

 _Noreturn void unbound_error(sl_v sym);

 _Noreturn void arity_error(int nargs, int c);

@@ -420,7 +421,7 @@

 extern sl_v sl_booleansym, sl_nullsym, sl_evalsym, sl_fnsym, sl_trimsym;

 extern sl_v sl_nulsym, sl_alarmsym, sl_backspacesym, sl_tabsym, sl_linefeedsym, sl_newlinesym;

 extern sl_v sl_vtabsym, sl_pagesym, sl_returnsym, sl_escsym, sl_spacesym, sl_deletesym;

-extern sl_v sl_errio, sl_errparse, sl_errtype, sl_errarg, sl_errmem;

+extern sl_v sl_errio, sl_errparse, sl_errtype, sl_errarg, sl_errmem, sl_errconst;

 extern sl_v sl_errdiv0, sl_errbounds, sl_err, sl_errkey, sl_errunbound, sl_erroom;

 extern sl_v sl_emptyvec, sl_emptystr;

--- a/src/vm.h

+++ b/src/vm.h

@@ -848,9 +848,9 @@

 	v = vec_elt(v, i);

 	assert(issym(v));

 	sl_sym *sym = ptr(v);

-	v = sp[-1];

-	if(!isconst(sym))

-		sym->binding = v;

+	if(sl_unlikely(isconst(sym)))

+		const_error(sym);

+	sym->binding = sp[-1];

 	NEXT_OP;

 }