[swift-evolution] [swift-evolution-announce] [Accepted and Focused Re-review] SE-0187: Introduce Sequence.filterMap(_:)

Drew Crawford drew at sealedabstract.com
Mon Nov 20 11:09:11 CST 2017

The typical case for this function in my code is the identity closure, that is

    let a: [Int?] = ...
    print(a.filterMap {$0})

filterMap is quite poor for this situation because *each* component in the term is inaccurate:

1.  While we are filtering in the "english sense", in Swift the word "filter" carries a specific meaning involving booleans.  In particular if `a` were of type `[Bool?]` I believe new Swift programmers would fail to predict the result
2.  This is not a map operation when used with the identity closure.  

Most of the discussion so far addresses 1 but not 2, e.g. mapSome, mapCompact improve dimension 1 but not dimension 2.

I support mapMaybe because from the choices so far it is the only one that seems to address both dimensions.

It can also be argued that the combined API itself is a mistake, e.g. that a distinct API for discarding optionals as from mapping would be superior.  But that may be too radical at this point.

On November 15, 2017 at 2:55:10 PM, John McCall (rjmccall at apple.com) wrote:

Hello, Swift Community!

The initial review of "SE-0187: Introduce Sequence.filterMap(_:)" ran through yesterday, November 14th, 2017.  The proposal is available here:


There was a significant amount of discussion, and people came down with reasonable arguments both for and against the proposal.  After reviewing that feedback, the core team feels that the central question is whether Swift benefits from overloading flatMap in this way.  There is a reasonable argument that an Optional is a sort of container, and therefore it makes sense to "flatten" that container into a surrounding container.  But Swift has resisted applying that interpretation in its library design; for example, you cannot directly iterate an Optional or append its contents to an Array.  In general, we feel that using different operations for working with Optionals tends to make code easier to both write and understand, especially given the existence of implicit optional promotion, which we cannot eliminate or easily suppress based on the context.  On reflection, we think it was a mistake to use the same name in the first place, and there is no better time to fix a mistake than now.

While we accept that this will cause some amount of "code churn" for developers when they adopt Swift 5, the required change is a simple rename that should be painless to automatically migrate.  Of course, sample code on the internet will become obsolete, but fix-its will easily update that code if pasted into a project, and the samples themselves (once corrected) should become clearer and easier to teach after this change, as is generally true when overloading is removed.

Accordingly, SE-0187 is accepted, at least as far as not calling the operation "flatMap".  We are re-opening the review until next Monday, November 20th, 2017, in order to have a focused discussion about the new name.  Names that seemed to gain some traction in the first review include:

  - filterMap, which has precedent in existing functional languages, as well as some popular Swift libraries, but which some people view as confusing

  - compactMap, which builds off the precedent of "compact" in Ruby

But please feel free to suggest a name other than these.


Reviews are an important part of the Swift evolution process.  All reviews should be sent to the swift-evolution mailing list at


or, if you would like to keep your feedback private, directly to me as the review manager.  When replying, please try to keep the proposal link at the top of the message:

Proposal link:

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What goes into a review?

The goal of the review process is to improve the proposal under review through constructive criticism and, eventually, determine the direction of Swift.

When writing your review, here are some questions you might want to answer in your review:

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As always, thank you for contributing to the evolution of Swift.

John McCall
Review Manager
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