The current Read instances for Float and Double look pretty iffy from an efficiency standpoint. Going through Rational is exceedingly weird: we have absolutely nothing to gain by dividing out the GCD, as far as I can tell. Then, in doing so, we read the digits of the integral part to form an Integer. This looks like a detour, and particularly bad when it has many digits. Wouldn't it be better to normalize the decimal representation first in some fashion (e.g., to 0.xxxxxxexxx) and go from there? Probably less importantly, is there some way to avoid converting the mantissa to an Integer at all? The low digits may not end up making any difference whatsoever.
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