Testing for NaN

The standard trick would be: isNaN d = (d /= d) This will not distinguish between quiet and signalling NaN's, on the other hand I wonder why on earth you might be getting signalling NaN's anyway. (A signalling NaN normally corresponds to "badly formatted float". Some compilers give float values initial values of signalling NaN, to make it clear they haven't been initialised; the only other reason I can think of why your program might start having signalling NaNs is because of a cast from some other type. I don't think any arithmetic operations should emit a signalling NaN, unless it was given one.)