2009/3/31 john skaller <[email protected]>
>
> On 31/03/2009, at 6:40 PM, Emmanuel Onzon wrote:
>
>
>> Having priorities allows this because you can insert one between
>> two others, Hmm. Note this required deleting a priority and adding two
>> more if they're not transitive .. hmmm ...
>>
>> Adding new priorities is not a problem, but deleting them
>> is currently not possible (except when going out of scope).
>> Could you give me an example where you need to delete
>> a priority to insert a new operator in the grammar ?
>>
>>
>
>
> Ooops, bad language .. I meant "priority relations".
>
Deleting priority relations is not possible either. (but adding
them is possible)
>
> Suppose you have + < ^ (addition less than
> exponentiation) and you want to add multiplication:
>
> + < *
> * < ^
>
> then since Dypgen relations are not transitive you have to *remove* the
> relation
>
> + < ^
>
> to build a strict chain.
No you don't have to remove it. Making a relations transitive
is not mandatory but it is not forbidden either.
You can even introduce the transitive closure of the relation
for a list of priorities with:
Relation [["p1";"p2";"p3";"p4"]]
If you remove + < ^ then you lose the precedence
of ^ over + and the expression:
a ^ b + c
become ambiguous.
> Felix doesn't need a strict chain usually. However there's
> a nasty case which should use one:
>
> e ^ - 2
>
> Say this shouldn't be allowed even though it unambiguously means
>
> e ^ ( -2 )
>
Here you argue that taking the opposite of a number
should not take precedence over the exponentiation.
Because:
>
> because
>
> - e ^ 2
>
> actually means
>
> - (e ^ 2)
>
> and not
>
> (-e) ^ 2
>
But this doesn't make the difference between the
precedence on the left and on the right, which dypgen
allows to do:
p_plus<p_times<p_exp<p_opp<p_nb
expr:
| expr(>p_exp) "^" expr(>=p_opp) { } p_exp
| "-" expr(>=p_nb) { } p_opp
| expr(>=p_times) "*" expr(>p_times) { } p_times
| expr(>=p_plus) "+" expr(>p_plus) { } p_plus
| number { } p_nb
> Felix actually allows e ^ -2, but the grammar is quite confusing, it allows
> this too:
>
> e ^ sin x
>
> which could mean
>
> (e ^ sin) x
>
> if you didn't know better. I've been caught by this myself. Anyhow, an
> example where
> one wants to *use* the non-transitivity, which requires removing a
> previously
> existing relation (but not the priority itself).
>
why? and between which priorities?
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