Skip to content

Stabilize core::convert::identity #57322

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 1 commit into from
Jan 14, 2019
Merged

Stabilize core::convert::identity #57322

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
5 changes: 1 addition & 4 deletions src/libcore/convert.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -55,7 +55,6 @@
/// Using `identity` to do nothing among other interesting functions:
///
/// ```rust
/// #![feature(convert_id)]
/// use std::convert::identity;
///
/// fn manipulation(x: u32) -> u32 {
Expand All @@ -69,7 +68,6 @@
/// Using `identity` to get a function that changes nothing in a conditional:
///
/// ```rust
/// #![feature(convert_id)]
/// use std::convert::identity;
///
/// # let condition = true;
Expand All @@ -86,14 +84,13 @@
/// Using `identity` to keep the `Some` variants of an iterator of `Option<T>`:
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I feel like this particularly would be better as flatten with an explicit specialisation for Option, rather than filter_map.

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I disagree.

First, I think that .filter_map(identity) is clearer wrt. intent (and also semantics). I can clearly tell that filtering is happening because it says so in the name. I know that .filter_map takes f: T -> Option<U> and with identity I fix T = Option<U> and thus I get back all the Somes. The .flatten() method doesn't tell me any of this, in particular you cannot tell without knowing that Option is being operated on what the semantics are. Thus the reasoning footprint is greater with .flatten().

Second, even tho .flatten() is more general than join :: Monad m => m (m a) -> m a I think the primary use of the method should be for monadic join and using it for other purposes will break some people's minds (e.g. mine).

Third, I think that we shouldn't hold up stabilization on this issue; we can resolve it in a subsequent PR or issue if you feel strongly.

///
/// ```rust
/// #![feature(convert_id)]
/// use std::convert::identity;
///
/// let iter = vec![Some(1), None, Some(3)].into_iter();
/// let filtered = iter.filter_map(identity).collect::<Vec<_>>();
/// assert_eq!(vec![1, 3], filtered);
/// ```
#[unstable(feature = "convert_id", issue = "53500")]
#[stable(feature = "convert_id", since = "1.33.0")]
#[inline]
pub const fn identity<T>(x: T) -> T { x }

Expand Down
2 changes: 0 additions & 2 deletions src/test/ui/rfc-2306/convert-id-const-with-gate.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -2,8 +2,6 @@

// compile-pass

#![feature(convert_id)]

fn main() {
const _FOO: u8 = ::std::convert::identity(42u8);
}