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699fdd47e8
compiler/switch_detection.m:
Sometimes a disjunction can be interpreted as a switch on more than
one variable. For example, the disjunction
(
A = x,
B = x,
...
;
A = x,
B = y,
...
;
A = y,
B = x,
...
;
A = y,
B = y,
...
)
can be viewed either as a switch on A (with a switch on B in each arm) or
as a switch on B (with a switch on A in each arm). However, if e.g. this
disjunction is in a require_complete_switch [B] scope, we *want* switch
detection to choose the second view.
Switch detection has traditionally tried to see if a disjunction is a
switch on each bound nonlocal variable of the disjunction in order
of those variable's variable numbers, which generates the right results
in this context only by accident. Change this so that inside a scope whose
subgoal is required to be a switch on a certain variable, we now look for
a switch on *that* variable first, so that if the disjunction *can* be
a switch on that variable, it *will be* a switch on that variable.
tests/valid/required_var.m:
A new test case to test the new capability.
tests/valid/Mmakefile:
Enable the new test case.
52 lines
1.5 KiB
Mathematica
52 lines
1.5 KiB
Mathematica
%---------------------------------------------------------------------------%
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% vim: ts=4 sw=4 et ft=mercury
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%---------------------------------------------------------------------------%
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%
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% Since A occurs earlier in the body of oops than B, the old switch detection
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% algorithm first tried to see whether the disjunction inside the
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% require_complete_switch scope was a switch on A, before it tried B.
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% Since the disjunction CAN be seen as a switch on A, it saw it that way,
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% which (after the changes committed on 2016 april 19) generated an error.
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%
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% This is a test for the fix: when a disjunction is a subgoal of a scope
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% that requires it to be a switch on a given variable, always start testing
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% whether the disjunction is a switch on a variable with the variable named
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% by the scope.
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:- module required_var.
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:- interface.
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:- type xy
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---> x
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; y(int).
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:- pred oops(xy::in, xy::in, int::out) is det.
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:- implementation.
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:- import_module int.
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oops(A, B, Result) :-
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% The compiler can see the disjunction either as a switch on A
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% with a switch on B in each arm, or as a switch on B with a switch on A
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% in each arm. Due to the require_complete_switch scope, it should prefer
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% the latter, even though A will have a lower variable number than B.
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require_complete_switch [B]
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(
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A = x,
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B = x,
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Result = 0
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;
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A = x,
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B = y(BN),
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Result = BN
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;
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A = y(AN),
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B = x,
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Result = AN
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;
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A = y(AN),
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B = y(BN),
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Result = AN + BN
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).
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