This works:
import std.range;
auto groupBy(alias func, R)(R values)
if (isInputRange!R)
{
alias K = typeof(func(values.front));
alias V = ElementType!R[];
V[K] grouped;
foreach(value; values) grouped[func(value)] ~= value;
return grouped;
}
unittest {
struct Test {
string a;
double b;
}
auto values = [Test( "a", 1 ), Test( "a", 2 ), Test( "b", 3 )];
auto grouped = values.groupBy!(a => a.a);
assert( grouped["a"].length == 2 );
assert( grouped["a"][1].b == 2 );
assert( grouped["b"].length == 1 );
assert( grouped["b"][0].b == 3 );
}
Atila
On Thursday, 16 October 2014 at 08:04:08 UTC, Edwin van Leeuwen
wrote:
I am trying to implement a groupBy function that groups by the
return type of a predicate. Currently I have to define the
returntype of the predicate for it to compile. Is there a way
to get the return type at compile time and use it.
The code:
V[K] groupBy( alias func, K, V )( V values )
{
V[K] grouped;
foreach ( value ; values ) {
grouped[func( value )] ~= value;
}
return grouped;
}
unittest {
struct Test {
string a;
double b;
}
auto values = [Test( "a", 1 ), Test( "a", 2 ), Test( "b", 3
)];
auto grouped = values.groupBy!( (a) => a.a, string );
assert( grouped["a"].length == 2 );
assert( grouped["a"][1].b == 2 );
assert( grouped["b"].length == 1 );
assert( grouped["b"][0].b == 3 );
}
So the above works, but I need to call it with:
values.groupBy!( (a) => a.a, string );
Ideally I would call it instead with:
values.groupBy!( (a) => a.a )
and it would infer that the template K needs to be a string,
since that is the return type of (a) => a.a.
Cheers,
Edwin
