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fac n = product [1..n]

fac n = if n<1 then 1 else n * (fac (n-1))

fac 0 = 1
fac n = n * fac (n-1)

fac n | n == 0 = 1
      | otherwise = n * (fac (n-1))

sub fac 
{
...
        return $n*fac($n-1);
    }
}