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]

>>5
Your signature is unclever and mathematically incorrect and doesn't conform to Prague standards.

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fact n = product $ takeWhile (/= 0) [n, n - 1 ..]