Post by Walter Bright via Digitalmars-dhttps://www.reddit.com/r/programming/comments/2ilsij/function_purity_and_immutable_data_structure/
A problem is caused by with(){} that creates a scope. If I want
to write the literal of an immutable array of many enumerations I
have to repeat the enum name many times, this is the opposite of
DRY:
void main() {
enum Seq { A, B, C, D, E, F, G, H }
immutable Seq[2][] seqs =
[[Seq.A, Seq.C], [Seq.A, Seq.D], [Seq.A, Seq.E],
[Seq.B, Seq.D], [Seq.B, Seq.E], [Seq.B, Seq.F],
[Seq.C, Seq.D], [Seq.D, Seq.E], [Seq.E, Seq.F],
[Seq.G, Seq.C], [Seq.G, Seq.D], [Seq.G, Seq.E],
[Seq.H, Seq.D], [Seq.H, Seq.E], [Seq.H, Seq.F]];
}
To avoid the repeating "Seq." I can use with() but now it can't
be immutable:
void main() {
Seq[2][] seqs;
with (Seq) seqs =
[[A, C], [A, D], [A, E],
[B, D], [B, E], [B, F],
[C, D], [D, E], [E, F],
[G, C], [G, D], [G, E],
[H, D], [H, E], [H, F]];
}
I can define it immutable only if I use a global name:
immutable Seq[2][] seqs;
pure nothrow @safe static this() {
with (Seq) seqs =
[[A, C], [A, D], [A, E],
[B, D], [B, E], [B, F],
[C, D], [D, E], [E, F],
[G, C], [G, D], [G, E],
[H, D], [H, E], [H, F]];
}
void main() {}
There is also this possibility, but for me it's too much hairy to
be used in real code:
void main() {
enum Seq { A, B, C, D, E, F, G, H }
immutable Seq[2][] seqs = {
with (Seq) {
Seq[2][] local =
[[A, C], [A, D], [A, E],
[B, D], [B, E], [B, F],
[C, D], [D, E], [E, F],
[G, C], [G, D], [G, E],
[H, D], [H, E], [H, F]];
return local;
}
}();
}
A solution seems to add a "static with" to D (usable at module
scope too), that doesn't create a new scope:
void main() {
enum Seq { A, B, C, D, E, F, G, H }
static with(Seq) immutable Seq[2][] seqs =
[[A, C], [A, D], [A, E],
[B, D], [B, E], [B, F],
[C, D], [D, E], [E, F],
[G, C], [G, D], [G, E],
[H, D], [H, E], [H, F]];
}
Bye,
bearophile