Any attempt to use (a variable set to) NaN .... should cause an exception.

That'd make it too easy ;-)

The mpfr library actually initialises the mpfr_t data type with a value of NaN.
It's something I sort of like - it implies (to me, anyway) that the developers, having no idea what value should be assigned initially, have quite appropriately assigned no value. It's up to the program to assign a value to the mpfr_t variable - and until that variable is assigned a value, it stays as NaN.

With mpfr, there's also the matter of ANSI/IEEE-754 compliance - which has a strong influence on the things it does.

If only the IEEE-754 standard had decreed "nans are to be consdered false in a boolean sense". (I don't think it does, but I'd love to be wrong about that ... otoh, I'd hate to learn that it decrees the converse.)

I think I might run this by the mpfr folk. It's quite OT there, of course, but knowing if there's some compelling reason to keep NaNs as "true" is something that's probably "right up their alley", so to speak.

Cheers,
Rob

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