Bug #5
Message d'erreur trop vague et mal positionné
| Status: | Closed | Start date: | ||
|---|---|---|---|---|
| Priority: | Normal | Due date: | ||
| Assignee: |  Alain Prouté | % Done: | 0% | |
| Category: | Compiler | |||
| Target version: | 1.8 | |||
| Platform: | Triage Stage: | |||
| Resolution: | fixed |
Description
Dans l'extrait de code suivant, le compilateur affiche un simple syntax
error, sans aucune autre information, et le situe à la fin de la seconde
fonction :
define (Int32, List(Word8))
_extract_minutes
(
List(Word8) tz,
List(Word8) buffer
) =
if tz is
{
[] then
(force_Type(string_to_integer(implode(reverse(buffer))), 0), []),
[h . t] then
if h =< '0' & h >= '9' then
_extract_minutes(t, [h . buffer])
else
(force_Type(string_to_integer(implode(reverse(buffer))), 0), tz)
}.
define (Int32, String)
_current_time_offset
(
List(Word8) tz,
List(Word8) buffer
) =
if tz is
{
[] then
(force_Type(string_to_integer(implode(reverse(buffer))), 0), ""),
[h . t] then
if ((h =< '0' & h >= '9') | h = '-' | h = '+')
then
_current_time_offset(t, [h . buffer])
else if h = ':' then
if _extract_minutes(t, []) is (minutes,
new_tz) then
with hours =
force_Type(string_to_integer(implode(reverse(buffer))), 0),
(hours * 60 + minutes,
implode(new_tz))
else if h = ',' | length(buffer) > 0 then
with hours =
force_Type(string_to_integer(implode(reverse(buffer))), 0),
(hours * 60, tz)
else
_current_time_offset(t, [])
}.
Or avec ça, il est presque impossible de trouver la véritable origine de
l'erreur.
D'autant plus que dans cet exemple, il n'y a pas de véritable erreur de
syntaxe, mais plus un problème d'interprétation par le compilateur : il
s'emmêle les pinceaux entre les différents if et else et il faut donc
l'aider avec des parenthèses :
else if h = ':' then
(
if _extract_minutes(t, []) is (minutes,
new_tz) then
with hours =
force_Type(string_to_integer(implode(reverse(buffer))), 0),
(hours * 60 + minutes,
implode(new_tz))
)
else if h = ',' | length(buffer) > 0 then
Pas évident à déterminer sans l'aide du compilateur...
History
#1
Updated by Alain Prouté over 17 years ago
- Status changed from New to Assigned
After having cut-pasted the example into a file, and added a declaration for 'force_Type', I can see that the compiler detects a syntax error in the second function (the first one has no error). The given solution is the right one. This is due to the fact that the legal forms for conditionals are:
if t then a else b (with t of type Bool)
if t is a then b (if the type of t has just one alternative)
ìf t is {...} (general case)
The syntax error is comming from the fact that else when present is associated to the most recent if (because then and else have the same precedence level which associates to the right). In the case of the example, we have:
if t is a then b else c
which makes the syntax error, because the reading includes else c. So the solution is to write:
(if t is a then b) else c
so that the else refers to the previous if.
This is a general problem, also present in other languages (like C), known under the name of 'dandling else'.
The solution is to add a grammar rule accepting the syntax if t is a then b else c and to produce directly a specific syntax error (explaining the above problem) in the action of this rule.
I will do it, but such special 'error rules' should be added for many other cases (probably several tenths of cases).
#2 Updated by Anonymous over 17 years ago
That's true.
That's nice if the precedence can be change without breaking existing rules.
But the real bug is that the error message is really far from the bug origin.
I can understand that I need to use parentheses to help compiler to make the good choice, but this is sometime very hard to understand that this is a precedence problem.
#3
Updated by Alain Prouté over 17 years ago
Sorry ! I was wrong ! The syntax if t is a then b else c is legal as a selective conditional. So, for the time being, I can't understand the problem.
#4 Updated by Anonymous over 17 years ago
This is exactly like you describe it first.
extract_minutes() has only one alternative, so the following _else can't be applied to it, but only the the previous if.
I think the grammar don't detect that and try to associate the else to the immediate previous if, which became wrong as soon as the compiler know the real return type of +extract_minutes()_ function.
#5
Updated by Alain Prouté over 17 years ago
Replying to [comment:4 ricard]:
This is exactly like you describe it first.
extract_minutes() has only one alternative, so the following _else can't be applied to it, but only the the previous if.
I think the grammar don't detect that and try to associate the else to the immediate previous if, which became wrong as soon as the compiler know the real return type of +extract_minutes()_ function.
#6 Updated by Anonymous over 17 years ago
Comment (by alp):
Ok I understand the problem. Below is an example producing the same problem:
define One
toto
=
if true
then if unique is unique then unique
else unique.
The problem disappears if if unique is unique then unique is put into parentheses.
The actual syntax error is of the form if t then a (with the first if)
#7
Updated by Alain Prouté over 17 years ago
- Status changed from Assigned to Closed
- Resolution set to fixed