I really donot know how to use `newtype` ?

then how to use `newtype` ?  Would you mind explaining the newtype in detail ?

Sinerely!

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fac n = foldr (*) 1 [1..n]

>>=1
Back to /haskell-cafe/, please.

>>=2
It's "GRUNNUR"! Not "GRUNNER"! That's not even a word! "GRUNNUR" is the standard!

>>=3
That's susceptible to stack pointer monadic overflow.