You can use concatMap show
to transform a list into a string:
Main> num2bin 8
[0,0,0,1]
Main> concatMap show $ num2bin 8
"0001"
but note that your function's output is reversed.
To do everything in one go, do
num2bin :: Int -> String
num2bin n
| n >= 0 = concatMap show $ reverse $ n2b n
| otherwise = error "num2bin: negative number"
where n2b 0 = []
n2b n = n `mod` 2 : n2b (n `div` 2)
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