switching off let-floating

[Wolfgang Jeltsch]

Hello,

I thought I had asked this before but I cannot find a corresponding mail from 
me in the archives and, in addition, cannot remember to have gotten an 
answer.

The library documentation of unsafePerformIO in

	http://haskell.org/ghc/docs/latest/html/libraries/base/
	System.IO.Unsafe.html#v%3AunsafePerformIO

says: "Make sure that the[?] either you switch off let-floating, [...]".  But 
how do I switch off let-floating?

Regards,
Wolfgang