Re: Pragma clash across modules: bignum and Math::BigFloat

[Anonymous Monk]

What's going on here is that bignum is meddling with the globals $Math::BigFloat::downgrade and $Math::BigInt::upgrade. It sets downgrade to Math::BigInt, which causes the BigFloat constructor to return BigInts instead, when they don't have a decimal point. Setting $Math::BigInt::upgrade makes BigInt::bsqrt() convert to BigFloat before taking the square root; the problem is, now BigFloat::bsqrt() is working on a copy and doesn't modify the original.

use bignum;
my $f = Math::BigFloat->new('2');
print $f, "\n";
print ref($f), "\n";
print $f->bsqrt(), "\n";
print $f, "\n";

output:

2
Math::BigInt
1.41421356237309504880168872420969807857
2

There's really no way for bignum to do the right thing. It would have to muck around with those globals every time the flow of control entered or left its lexical scope. And the automatic upgrade feature was just a bad idea from the start. Solution: Don't use bignum, I'm afraid.

[LanX]

> bignum is meddling with the globals $Math::BigFloat::downgrade and $Math::BigInt::upgrade.

I haven't checked in detail yet (the code is a bit obscure) but this sounds reasonable.

But I disagree when you say

> There's really no way for bignum to do the right thing. It would have to muck around with those globals every time the flow of control entered or left its lexical scope. And the automatic upgrade feature was just a bad idea from the start. Solution: Don't use bignum, I'm afraid.

Athanasius point was not why bignum fails but why the use of pragma bignum is effecting a completely other package, since pragmas are guaranteed to be limited to a scope.

It's rather a limitation of BigFloat and BigInt to use global variable which keep their state even if the scope is left.

perlpragma shows a clear mechanism to limit effects to the scope using a pragma with the help of the hinthash %H .

Since "bignum is just a thin wrapper around various modules of the Math::BigInt family" those modules should be modified in a way to check a delegated hinthash when called from a pragma. (N.B. they share the same authors)

Hence a bug, in my humble opinion.

Anyway using globals for such profound effects is never a good idea.

update

according to perlglossary#P

• pragma
  A standard module whose practical hints and suggestions are received (and possibly ignored) at compile time. Pragmas are named in all lowercase.

I'm not sure if bignum qualifies as a pragma, because of it's rather runtime effects.

[Anonymous Monk]

Pragmas are named in all lowercase.

But not everything that is named in all lowercase is a pragma. Take use lib, for instance. Not a pragma; it globally changes @INC. bignum tried to be a pragma, but gave up halfway.

[LanX]

The definition is indeed fuzzy.

But in the case of lib I'd opt for pragma because it's a compile time effect and the globality is intended.

> bignum tried to be a pragma, but gave up halfway.

I agree.

For instance a Tie::Scalar on $upgrade and $downgrade to a routine checking the hinthash could do without even changing BigFloat and family.