Mercury inherited its original system of operator priorities from Prolog,
because during its initial development, we wanted to be able execute
the Mercury compiler using NU-Prolog and later SICStus Prolog.
That consideration has long been obsolete, and now we may fix the
design error that gifted Prolog with its counter-intuitive system
of operator priorities, in which higher *numerical* operator priorities
mean lower *actual* priorities. This diff does that.
library/ops.m:
Change the meaning of operator priorities, to make higher numerical
priorities mean also higher actual priorities.
This semantic change requires corresponding changes in any other module
that uses ops.m. To force this change to occur, change the type
representing priorities from being a synonym for a bare int to being
a notag wrapper around an uint.
The old "assoc" type had a misleading name, since it is related to
associativity but is not itself a representation of associativity.
Its two function symbols, which used to be just "x" and "y", meant that
the priority of an argument must be (respectively) greater than,
or greater than equal to, the priority of the operator. So rename
x to arg_gt, y to arg_ge, and assoc to arg_prio_gt_or_ge.
Rename the old adjust_priority_for_assoc predicate to min_priority_for_arg,
which better expresses its semantics. Turn it into a function, since
some of its users want it that way, and move its declaration to the
public part of the interface.
Add a method named tightest_op_priority to replace the use of 0
as a priority.
Rename the max_priority method as the loosest_op_priority method.
Add a method named universal_priority to replace the use of
max_priority + 1 as a priority.
Add a function to return the priority of the comma operator,
to allow other modules to stop hardcoding it.
Add operations for comparing priorities and for incrementing/decrementing
priorities.
Change the prefix on the names of the predicates that take op_infos
as inputs from "mercury_op_table_" to "op_infos_", since the old prefix
was misleading.
Add a note on an significant old problem with an exported type synonym.
library/mercury_term_parser.m:
Conform to the changes above.
Delete unnecessary module qualifiers, since they were just clutter.
Add "XXX OPS" to note further opportunities for improvement.
library/pprint.m:
Conform to the changes above.
Rename a function to avoid ambiguity.
library/pretty_printer.m:
library/stream.string_writer.m:
library/string.to_string.m:
library/term_io.m:
Conform to the changes above.
library/string.m:
Add a note on an significant old problem.
NEWS:
Announce the user-visible changes.
tests/hard_coded/bug383.m:
Update this test case to use the new system of operator priorities.
tests/hard_coded/term_io_test.{m,inp}:
Fix white space.
extras/old_library_modules/old_mercury_term_parser.m:
extras/old_library_modules/old_ops.m:
The old contents of the mercury_term_parser and ops modules,
in case anyone wants to continue using them instead of updating
their code to use their updated equivalents.
samples/calculator2.m:
Import the old versions of mercury_term_parser and ops.
Estimated hours taken: 5
Branches: main
Fix a bug reported by Doug Auclair.
library/ops.m:
Change the Mercury arg priority to 999. It was previously set to
1201, which allowed arguments and list elements to be parsed the
way we intended. However, this had the unintended side effect that
valid terms were written out using incorrect syntax.
library/parser.m:
Place the workaround here, where arguments and list elements are
read.
Note that this is still technically buggy, because we effectively
allow syntax which should be illegal according to the operator
precedence table. But the proper fix for this involves moving to
a BNF style parser, so that is left for later work.
tests/hard_coded/Mmakefile:
Run the term_io_test case twice, checking it against the expected
output each time. The first time it is run with the supplied input
file. The second time it is run with the expected output as input --
this ensures that terms are read in the same way that they are
written out.
tests/hard_coded/term_io_test.m:
Read input from stdin rather than a fixed file.
tests/hard_coded/term_io_test.inp:
Add additional input terms.
tests/hard_coded/term_io_test.exp:
tests/invalid/*.err_exp:
Update the expected output for this and other test cases.
