On Thursday, 21 April 2022 at 21:02:47 UTC, JG wrote:
Hi,
Could someone possibly help me to understand why the commented
line doesn't compile?
```d
import std;
struct MapResult(R,F)
{
R r;
const F f;
auto empty() { return r.empty; }
auto front() { return f(r.front); }
void popFront() { r.popFront; }
auto save() { return typeof(this)(r.save,f); }
}
auto myMap(alias f, R)(R r) {
return MapResult!(R,typeof(f))(r,f);
}
void main()
{
int function(int) f = x=>2*x;
iota(10).myMap!f.writeln;
//iota(10).myMap!(x=>2*x).writeln; <--- Why doesn't this
compile?
}
```
(I do know that Phobos's map works differently with better
performance).
Thank you to all for the great replies. To fix it one could do:
```d
import std;
struct MapResult(R,F) {
R r;
const F f;
auto empty() { return r.empty; }
auto front() { return f(r.front); }
void popFront() { r.popFront; }
auto save() { return typeof(this)(r.save,f); }
}
auto myMap(alias f, R)(R r) {
static if(__traits(compiles,f!(typeof(R.init.front)))) {
auto fun = f!(typeof(R.init.front));
return MapResult!(R,typeof(fun))(r,fun);
} else {
return MapResult!(R,typeof(f))(r,f);
}
}
void main()
{
int function(int) f = x=>2*x;
iota(10).myMap!f.writeln;
iota(10).myMap!(x=>2*x).writeln;
}
```
In response to the change to "alias", which has several upsides
including faster code. I would note it also has some downsides
including
every lambda produces a new type so that (at the moment) the
following assert
holds:
```d
auto r = iota(10).map!(x=>x+1);
auto s = iota(10).map!(x=>x+1);
assert(!is(typeof(r)==typeof(s)));
```
