From 15cc916333d823ae11f2da9a0b78d5cd1de9f995 Mon Sep 17 00:00:00 2001
From: Markus Triska <triska@metalevel.at>
Date: Sat, 14 May 2022 08:09:01 +0200
Subject: [PATCH 1/2] FIXED: can_be(chars, [a,X]), i.e., if variables occur as
 elements

This should simplify 2f3de51e554629d2e7b4ea023c9ef181c240692c and
other cases like it.
---
 src/lib/error.pl | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/lib/error.pl b/src/lib/error.pl
index 68d7d3ee..a7cc4a5a 100644
--- a/src/lib/error.pl
+++ b/src/lib/error.pl
@@ -128,7 +128,11 @@ can_be(Type, Term) :-
 can_(integer, Term) :- integer(Term).
 can_(atom, Term)    :- atom(Term).
 can_(character, T)  :- character(T).
-can_(chars, Ls)     :- '$is_partial_string'(Ls).
+can_(chars, Ls)     :-
+        (   '$is_partial_string'(Ls) -> true
+        ;   can_be(list, Ls),
+            can_be_chars(Ls)
+        ).
 can_(list, Term)    :- list_or_partial_list(Term).
 can_(boolean, Term) :- boolean(Term).
 can_(term, Term)    :-
@@ -137,6 +141,12 @@ can_(term, Term)    :-
         ;   type_error(term, Term, can_be/2)
         ).
 
+can_be_chars(Var) :- var(Var), !.
+can_be_chars([]).
+can_be_chars([X|Xs]) :-
+        can_be(character, X),
+        can_be_chars(Xs).
+
 list_or_partial_list(Ls) :-
         '$skip_max_list'(_, _, Ls, Rs),
         (   var(Rs) -> true

From a5e72679bcc615b737aa32dee5b24fdf4f426619 Mon Sep 17 00:00:00 2001
From: Markus Triska <triska@metalevel.at>
Date: Sat, 14 May 2022 09:52:34 +0200
Subject: [PATCH 2/2] prioritize type errors over instantiation errors

Example:

    ?- must_be(chars, [a,X,cc]).
       error(type_error(character,cc),can_be/2).

See https://github.com/mthom/scryer-prolog/pull/1474#issuecomment-1126664368
---
 src/lib/error.pl | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/lib/error.pl b/src/lib/error.pl
index a7cc4a5a..87b3ff46 100644
--- a/src/lib/error.pl
+++ b/src/lib/error.pl
@@ -1,5 +1,5 @@
 /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-   Written September 2018 by Markus Triska (triska@metalevel.at)
+   Written 2018-2022 by Markus Triska (triska@metalevel.at)
    I place this code in the public domain. Use it in any way you want.
 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
 
@@ -48,6 +48,7 @@ must_be_(integer, Term) :- check_(integer, integer, Term).
 must_be_(atom, Term)    :- check_(atom, atom, Term).
 must_be_(character, T)  :- check_(error:character, character, T).
 must_be_(chars, Ls) :-
+        can_be(chars, Ls), % prioritize type errors over instantiation errors
         must_be(list, Ls),
         (   '$is_partial_string'(Ls) ->
             % The expected case (success) uses a very fast test.