I just found out about guarded equations in Haskell, here's an example:

abs n | n >= 0 = n
      | otherwise = -n

it basically says that function abs returns n if n >= 0, otherwise it returns -n.

In the book it says that the the standard prelude otherwise is defined like this: otherwise = True, why is that?

CodePudding user response：

The reason why otherwise exists - and why it is defined the way it is - is purely for stylistic purposes. The example you gave could be just as validly, writtent like so:

abs n | n >= 0 = n
      | True = -n

But otherwise just reads more nicely. In particular it reads like a mathematical definition.

CodePudding user response：

It is defined as True, because the otherwise case always has to succeed.

For example, we can write:

signum n | n < 0 = -1
         | n == 0 = 0
         | otherwise = 1

and in this case Haskell checks if n < 0 is True, then, if it is False it checks if n == 0 is True, and then if it is False it checks if otherwise is True. As you can see, it has to go case by case until it finds one that is True, this is why otherwise has to be True (in order to succeed, when everything else is False).