Heinrich Apfelmus apfelmus at quantentunnel.de
Sat Mar 14 04:39:18 EDT 2009

```Colin Paul Adams wrote:
>>>>>> "Heinrich" == Heinrich Apfelmus <apfelmus at quantentunnel.de> writes:
>
>     Heinrich> Abstraction is the one driving force for very short
>     Heinrich> names. For example, take the definition of foldr
>
>     Heinrich>    foldr f z [] = z foldr f z (x:xs) = f x (foldr f z
>     Heinrich> xs)
>
>     Heinrich> Since this function is polymorphic, so f , z and the xs
>     Heinrich> can be anything, using more "descriptive" variable names
>     Heinrich> is simply not possible; the key point of fold is its
>     Heinrich> generality.
>
> Wouldn't unit be a better descriptive name than z?

I have never heard of a unit in relation to  fold , I'm afraid. Monoids
and groups have units, as do physicists and a few other mathematical
structures.

While  z  is indeed quite often the unit of a monoid, for instance in

sum     = foldr (+) 0
product = foldr (*) 1
concat  = foldr (++) []

it doesn't have to be the unit of a monoid.

Regards,
apfelmus

--
http://apfelmus.nfshost.com

```