Haskell: undefined

Haskell Prelude.hs file defined undefined like below.

undefined        :: a

undefined        =  error "Prelude.undefined"

For example,

Sample.hs

factorial :: Integer -> Integer
factorial num
    | (num < 0) = undefined
    | (num == 0) = 1
    | (num > 0) = num * factorial(num-1)

Prelude> :load Sample.hs
[1 of 1] Compiling Main             ( Sample.hs, interpreted )
Ok, modules loaded: Main.
*Main> 
*Main> factorial 5
120
*Main> 
*Main> factorial (-10)
*** Exception: Prelude.undefined

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