[Haskell-cafe] Please add a method for optimized concat to the Semigroup class
gale at sefer.org
Tue May 3 14:16:54 CEST 2011
Stephen Tetley wrote:
>> Does it have an obvious default implementation, bearing in mind it we
>> might really want a total function?
>> sconcat  = error "Yikes - I wish this was total!"
>> sconcat [a] = a
>> sconcat (a:as) = a <> sconcat as
Holger Siegel wrote:
> You have to provide the "neutral" element by yourself:
> a <>>  = a
> a <>> (b:bs) = a <> b <>> bs
Yes, I think that would be the best interface.
At first glance, one would be tempted to do something
like returning a Maybe, as is often done in these kinds
of cases. But here, the whole point of Semigroup is that
we don't know what to do when the list is empty, so getting
a Nothing result in that case is unhelpful.
To illustrate the point, let's look at the conversion between
those two approaches:
sconcatNonempty x xs = fromJust . sconcat $ x : xs
sconcatMaybe (x:xs) = Just $ sconcat x xs
sconcatMaybe _ = Nothing
I would much rather write sconcatMaybe when needed
than to have to write unsafe code like sconcatNonempty.
Presumably it's actually safe, since you would expect
implementations to provide a result whenever the list
is non-empty. But the type no longer provides that
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