Comment by rspeele
2 days ago
Map and filter usually have only one arg and if they have 2, the 2nd is almost always a 0-based index. They look identical in most languages, even when Microsoft chooses to call them Select and Where.
Reduce has an accumulator and a 2-arg function and languages are not very consistent amongst each other as to whether it's reduce(initial_acc, callback(acc, elem)) or reduce(callback(acc, elem), initial_acc) or reduce(callback(elem, acc), initial_acc) or what.
Hard to remember. Also some languages have a version of reduce that doesn't take an initial accumulator at all, which is just a footgun waiting for you to hit an empty collection. Also ALSO, the accumulator can easily become awkward in languages that don't support anonymous types or don't support easy mutation of an anonymous type record. Which is most of them!
Haskell got this right. You have foldr (right fold) and foldl' (left fold), and the order of the callback is opposite. If you do a left fold, then the initial accumulator is applied on the left; if you do a right fold, then the initial accumulator is applied on the right.
The mnemonic here is that the folding function (aka the callback) replaces the comma.
I find this slightly easier to remember than other languages. In contrast most other languages do not simultaneously provide a left fold and a right fold, so they do not consider this aspect, making things more difficult to remember.
That said I totally agree this requires more brainpower to read and write than map or filter. For this reason I have sometimes refactored code to use foldMap instead of foldr or foldl', so one no longer needs to think of the direction of the fold or the order of arguments.
When the accumulator isn't the second argument in a fold, left or right, it feels wrong and I waste some time cursing whomever made a silly mistake like getting the order wrong.
Luckily the functional languages I use the most are sane in that respect.
It's still a complex and more abstract function than map or filter. Those do a single thing that's easy to grasp. reduce/fold can be easily abused to duplicate the effect of most other collection functions, at the cost of making the code less readable. Although for slightly-too-clever people, that could mean you only need to know one function instead of all of them.
But it hurts readability. If you're going to do it, at least don't use it anonymously, but give it a name that clearly describes what's going on.
But even then, there can be hidden performance traps. I've often seen javascript that used reduce and created the new accumulator by using a spread on the old accumulator and adding the new one: `[...acc, newValue]`. But that spread is another iteration inside a loop, turning it from O(n) to O(n^2). A for loop where you append it is much faster.
Since you mention mnemonics, here are the mnemonics I use to remember the (symmetrical) differences between foldr and foldl
https://arialdomartini.github.io/fold-mnemonics
In other words, look at the types. The type of the folding function (the first argument) indicates how each fold works.
That's what I tend to do, but since foldr/foldl' is so ubiquitous in Haskell it would be nice if I could just remember the argument order of the callback. kccqzy's explanation (in particular "it replaces the comma") might just help me do that :)
I admit that I have always looked at an explanation like yours with x1,...,xn when using fold because I could never keep it straight in my mind.
In GNU Guile `reduce` is described as a special case of `fold`, where the first element is suitable to be used as initial value, while `fold` is more general and lets you specify another initial value. I think that makes a lot of sense.
In some programming languages with RPN you can avoid this problem, because it makes sense to put it in the stack as the initial value, and then you can as easily have multiple initial values; and then the callback function can read that from the stack that you had put there, like anything else you will push into the stack to read it back later. For example, in PostScript you can write something like:
However, this is not as good if you want to use the first element as the initial value instead, but still it can be done but it is then not as simple (unlike in programming languages that do not use RPN but instead with function call with arguments, in which case it might be simpler).
I guess names as SELECT and WHERE are like SQL (although SQL works differently than other programming langauges).
Also reduce is a weird name.
What about fold?
In rust iterators there's both fold (you supply the initial value) and reduce (it uses the first element as the initial value, doesn't work on empty iterators)
https://doc.rust-lang.org/std/iter/trait.Iterator.html#metho...
https://doc.rust-lang.org/std/iter/trait.Iterator.html#metho...
Why? It does, after all, reduce a collection to a single value.
The resulting value can be anything you want. You can turn a list into a tree, or another list.
Bad example:
Reduces the list to another list three times as long.
It's a reduction in the sense of a transformation (also often seen in complexity theory), not in the "this makes this smaller" everyday usage that I think about first.
I suppose it's just so ... reductive, you know?
In the book "Simply Scheme", map is "every", filter is "keep", and reduce is "accumulate".
> … to call them Select and Where.
While map is a great name, I always struggle to remember if ‘filter’ keeps elements that match the condition or removes them.
I mean, it’s like a colander: you filter noodles and water, but which one do you keep? The noodles, right? But, replace noodles with tea and now you want to keep the water part.
Naming is hard I guess.
There's always the Ruby strategy of just making all the names work. `select` and `filter` are buddies and you can use whichever you want or even go back and forth. Not a fan of `reduce`? That's fine, `inject` has got your back. Miss getting to type `collect` from Java or Rust? Don't worry, just use it instead of `map`, it's the same thing.
I believe Ruby uses names inspired by SmallTalk.
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Talking about un-guessable, misleading function names,
C++ std::remove.
I would never have guessed what it does exactly. (It moves elements that match the filter to the front, and moves the end-marker forward. Leaves all the elements in the collection. You need to erase them yourself. )
It returns the new end marker
Oh it's a bit like unordered-delete when using an arena. I guess I would have expected an ordered-delete instead
I've never run into a generic "filter" function which keeps only the non-matching elements.
Smalltalk has #reject: which does that. You could, of course, just wrap a not around the test in the closure, but sometimes reject with a well-named predicate is easier to read.
bsnpApproved := tvShows reject: [ :eachShow | eachShow hasNaughtyContent ].
Common Lisp's filter is remove-if which works this way.
(It also has remove-if-not but that's deprecated and if you use it your code smells.)
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The filter keeps the tea... it's just that you then lift the filter out of the cup, carrying the tea with it. Flip your brain around to see it from that direction and it might help you with the mnemonics.
if you had parameter names maybe it might help?
`filter(where:)` like in swift...?
Doesn't seem to help the ambiguity to me.
Maybe those two could be filter_for (the “where” case) and filter_out.
Kotlin has filter and filterNot (it also has separate "reduce" and "fold" functions, dependingon whether you want to specify an initial accumulator value or not)
If you're making tea with a colander something is very wrong ;)
I was thinking an apt analogy might be making stock -- you filter out all the solid food you don't want to keep in the liquid.
And it's a doubly-good analogy, because I have occasionally gotten that confused in real-life as well. Twice in the past ten years I've had a stock boil away for three hours, and then set a colander in the sink and poured it through, only to watch my beautiful stock swirl down the drain because motor-memory made me forget that I wasn't draining pasta but should have put the colander in a bowl...
depends on the size of the sieve, but sometimes one does cook a whole stewpot of tea at once (f.e. in canteen)
> While map is a great name, I always struggle to remember if ‘filter’ keeps elements that match the condition or removes them.
In Common Lisp both functions exist, under the names `remove-if` and `remove-if-not`.
select/reject (Ruby)
It doesn't help that fold/reduce often have different orders depending on the ecosystem. Every few months when I have a reason to reach for `fold` in nutshell I forget that it has the next element as the first arg instead of the second, which is what I'm used to from Rust. I guess I should just be happy I don't need to specify which direction I want like in OCaml.
> Map and filter usually have only one arg and if they have 2, the 2nd is almost always a 0-based index. They look identical in most languages, even when Microsoft chooses to call them Select and Where.
I don't understand. Map takes input of type a and size n and returns output of type b and size n.
Filter takes input of type a and size n and returns output of type a and size ≤ n.
They look nothing alike?
Exactly, and sometimes you also get the indez as argument of the function. `reduce(acc,(acc,elem,idx)=>…)`
and in many case the accumulator is a tuple, and in many cases you need to know the length of the collection ( like average)
all in all, it’s a lot just to avoid a for loop.
An IDE can fix that
Meh, if you need a computer program to understand an API its a bad api.
APIs should make sense inherently. An IDE can band-aid a bad design, but that doesn't make it a good design.
There are lots of APIs where the order of argument isn't obvious, it doesn't mean they're bad designs
That's why I only program in punched cards. If you need a visual editor you're a bad programmer.
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