[Haskell-cafe] A functor for two Peano systems

Aaron VonderHaar gruen0aermel at gmail.com
Mon Apr 5 23:36:41 UTC 2021

> That approach isn't working for you, and it's trying the patience of the
cafe. (These are sub-beginner level q's, please use the Beginners'

According to https://www.haskell.org/mailing-lists/ , the purpose of
haskell-cafe is "General Haskell questions; extended discussions. Forum in
which it’s acceptable to ask anything, no matter how naive, and get polite
replies."  The OP's questions and efforts are more than appropriate here,
and they should expect no less than polite, if not helpful, responses here.

AntC, your replies are not at all polite, and you seem to be misinformed of
the purpose of this mailing list.  Please help us maintain a more
appropriate environment here in the future, and I would ask the
haskell-cafe moderators to assist in this as well.  If you want to direct
someone to a different forum, please do so politely and without insulting

--Aaron V.

On Mon, Apr 5, 2021 at 4:19 PM Anthony Clayden <anthony_clayden at clear.net.nz>

> > This whole exercise is my attempt to translate *The Littler MLer* and
> its last chapter where they go into functors.
> That approach isn't working for you, and it's trying the patience of the
> cafe. (These are sub-beginner level q's, please use the Beginners' forum.
> And I'd forgotten that's what you're doing. Your first post in this thread
> should have said you're talking about ML functors.
> https://stackoverflow.com/questions/2030863/in-functional-programming-what-is-a-functor#:~:text=In%20the%20ML%20family%20of,programmers%20have%20difficulty%20with%20it. says
> "most beginning programmers have difficulty with [functors]".)
> I suggest you first go through a Haskell tutorial, and get your knowledge
> secure of idiomatic Haskell. Even so, I just don't believe you're coming
> from idiomatic ML. Unless 'The Little MLer' is giving it as obfuscated
> code, and the objective is to de-obfuscate it.
> I'd code your `plus2` with pattern-matching:
> > plus2 :: MyNum -> MyNum -> MyNum
> > plus2 MNZero m = m
> > plus2 (OneMoreThan n') m = OneMoreThan $ plus2 n' m
> Your test for `(x == MNZero)` inside helper function `pCessor` is useless:
> flow-of-control doesn't take the outer `else` branch unless `x` (i.e. `n`)
> is _not_ equal `MNZero`.
> Similar redundant code in the case for `Int`. But that has bigger
> problems, as Olaf points out: `Int`s can be negative. So if `plus` is
> called with a negative `n`, it'll call `pCessor` repeatedly until stack
> overflow (or numeric underflow).
> AntC
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