Bug 16034 – map should be possible with a reference only

the following doesn't work: void main() { struct One { int entry = 1; @disable this(this); } One[] ones = [One(), One()]; import std.algorithm.iteration: map; import std.algorithm.comparison: equal; assert(ones.map!`a.entry + 1`.equal([2, 2])); } foo.d(12): Error: template std.algorithm.iteration.map!"a.entry + 1".map cannot deduce function from argument types !()(One[]), candidates are: /usr/include/dlang/dmd/std/algorithm/iteration.d(450): std.algorithm.iteration.map!"a.entry + 1".map(Range)(Range r) if (isInputRange!(Unqual!Range)) (It doesn't matter whether string or real lambda are used)

Underlying issue: isInputRange!(One[]) is false.

IMO, the language limitation that ref is not part of the type (but is instead only a storage class), prevents making ranges work (among other things) work with non-copyable types. If that wasn't the case ElementType!(One[]) could return ref(One), and you could use map with a lambda whose paramter is ref(One).

The problem is not the lambda's parameter, but its return value. Essentially, we would like the lambda to return by `ref` when `r.front` returns by `ref`, so that it does not create an unnecessary copy. For a normal function, the solution would be to use `auto ref`, but the lambda syntax does not support this (issue 21243), so we must either fix that issue first or find a workaround. One workaround is to use a string mixin, like this: private enum getFrontLambda = q{(return ref R r) => r.front}; enum bool isInputRange(R) = /* ... */ && ( is(typeof(mixin("ref ", getFrontLambda))) || is(typeof(mixin(getFrontLambda))) ) /* ... */ This essentially replicates the logic of inferring `ref` in user code: attempt to compile with `ref`, then fall back to non-`ref` if it fails.

@jamesragray updated dlang/phobos pull request #8573 "Fix issue 16034: map should be possible with a reference only" fixing this issue: - Fix issue 16034: map should be possible with a reference only https://github.com/dlang/phobos/pull/8573

dlang/phobos pull request #8573 "Fix issue 16034: map should be possible with a reference only" was merged into master: - 41959deae627dba93a88f6c59ec9200d35684106 by james: Fix issue 16034: map should be possible with a reference only https://github.com/dlang/phobos/pull/8573