Maybe this should work: import std.algorithm: reduce; struct Foo { immutable int x; } void main() { auto data = [Foo(0)]; reduce!((a, b) => a)(data); } DMD 2.063alpha gives: ...\dmd2\src\phobos\std\algorithm.d(702): Error: cannot modify struct result Foo with immutable members ...\dmd2\src\phobos\std\algorithm.d(723): Error: template instance test.main.reduce!(__lambda3).reduce!(Foo,Foo[]) error instantiating test.d(7): instantiated from here: reduce!(Foo[]) test.d(7): Error: template instance test.main.reduce!(__lambda3).reduce!(Foo[]) error instantiating
Using DMD32 v2.061 on Windows 32bit, below doesn't work too. I think this is the same issue: ----- import std.stdio; import std.algorithm : sort; struct S { immutable int x; } void main() { auto arr = [S(0), S(2), S(1)]; sort!((a, b) => a.x < b.x)(arr); arr.writeln(); } ----- algorithm.d(1814):Error: cannot modify struct lhs S with immutable members algorithm.d(1815): Error: cannot modify struct rhs S with immutable members algorithm.d(7972): Error: template instance std.algorithm.swap!(S) error instantiating algorithm.d(7908): instantiated from here: swapAt!(S[]) algorithm.d(7993): instantiated from here: getPivot!(__lambda3, S[]) algorithm.d(7712): instantiated from here: quickSortImpl!(__lambda3, S[]) main.d(12): instantiated from here: sort!(__lambda3, cast(SwapStrategy)0, S[]) algorithm.d(7908): Error: template instance std.algorithm.swapAt!(S[]) error instantiating algorithm.d(7993): instantiated from here: getPivot!(__lambda3, S[]) algorithm.d(7712): instantiated from here: quickSortImpl!(__lambda3, S[]) main.d(12): instantiated from here: sort!(__lambda3, cast(SwapStrategy)0, S[]) algorithm.d(7993): Error: template instance main.main.getPivot!(__lambda3, S[]) error instantiating algorithm.d(7712): instantiated from here: quickSortImpl!(__lambda3, S[]) main.d(12): instantiated from here: sort!(__lambda3, cast(SwapStrategy)0, S[]) algorithm.d(7946): Error: cannot modify struct r[j] S with immutable members algorithm.d(7949): Error: cannot modify struct r[j] S with immutable members algorithm.d(8028): Error: template instance main.main.optimisticInsertionSort!(__lambda3, S[]) error instantiating algorithm.d(7712): instantiated from here: quickSortImpl!(__lambda3, S[]) main.d(12): instantiated from here: sort!(__lambda3, cast(SwapStrategy)0, S[]) algorithm.d(7712): Error: template instance main.main.quickSortImpl!(__lambda3, S[]) error instantiating main.d(12): instantiated from here: sort!(__lambda3, cast(SwapStrategy)0, S[]) main.d(12): Error: template instance main.main.sort!(__lambda3, cast(SwapStrategy)0, S[]) error instantiating
(In reply to comment #0) > Maybe this should work: > > > import std.algorithm: reduce; > struct Foo { > immutable int x; > } > void main() { > auto data = [Foo(0)]; > reduce!((a, b) => a)(data); > } > > > > DMD 2.063alpha gives: > > ...\dmd2\src\phobos\std\algorithm.d(702): Error: cannot modify struct result > Foo with immutable members > ...\dmd2\src\phobos\std\algorithm.d(723): Error: template instance > test.main.reduce!(__lambda3).reduce!(Foo,Foo[]) error instantiating > test.d(7): instantiated from here: reduce!(Foo[]) > test.d(7): Error: template instance test.main.reduce!(__lambda3).reduce!(Foo[]) > error instantiating I think it is acceptable to require that whatever the type being reduced be assignable? It would be more friendly if it asserts though.
(In reply to comment #1) > Using DMD32 v2.061 on Windows 32bit, below doesn't work too. > I think this is the same issue: > ----- > import std.stdio; > import std.algorithm : sort; > > struct S > { > immutable int x; > } > > void main() > { > auto arr = [S(0), S(2), S(1)]; > sort!((a, b) => a.x < b.x)(arr); > arr.writeln(); > } > ----- Indeed. But in this case, `arr` is *simply* not sortable: Not without mutating the immutable. This is fixed now though: Error: template std.algorithm.sort cannot deduce function from argument types !((a, b) => a.x < b.x)(S[])
Yes, I think this is invalid, because the documentation states: //---- Then, for each element $(D x) in $(D range), $(D result = fun(result, x)) gets evaluated. //---- And here, the type is not assignable, even when unqualed. That said, a recursive "divide and conquer" fold could support such a case. I'm leaving this open as en ER.
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