On Friday, 16 October 2015 at 15:48:59 UTC, Edwin van Leeuwen
wrote:
Just wondering if anyone has any tips on how to solve/avoid
"cannot deduce function from argument types" when relying on
template programming.
I run into these problems all the time. Current one was when I
tried:
```
auto ys = NumericLabel(groupedAes.front.map!((t)=>t.y));
```
NumericLabel is a pretty minimal templated InputRange, with as
only requirement on the argument that it is also an InputRange.
I then get the following compile error:
```
source/ggplotd/geom.d(83,35): Error: struct
ggplotd.aes.NumericLabel cannot ded
uce function from argument types !()(MapResult!(__lambda2,
FilterResult!(__lamb
da2, Aes!(double[], "x", double[], "y", string[], "colour")))),
candidates are:
source/ggplotd/aes.d(515,1): ggplotd.aes.NumericLabel(T)
if (isInputRang
e!T)
```
As far as I know MapResult always returns an input range and as
you can see there is only one candidate of NumericLabel, so to
be honest it looks relatively straightforward to me.
Now I can define the type specifically (and/or typeof) but it
seems like there should be a better way.
In this case typeof isn't even happy:
```
source/ggplotd/geom.d(84,17): Error: constructor
ggplotd.aes.NumericLabel!(MapResult!(__lambda2,
FilterResult!(__lambda2, Aes!(string[], "x", string[], "y",
string[], "colour")))).NumericLabel.this (MapResult!(__lambda2,
FilterResult!(__lambda2, Aes!(string[], "x", string[], "y",
string[], "colour"))) range) is not callable using argument
types (MapResult!(__lambda3, FilterResult!(__lambda2,
Aes!(string[], "x", string[], "y", string[], "colour"))))
```
If we look closely, it expects a __lambda2 as the MapResult
argument, but it gets a __lambda3.
I am able to work around it by converting the mapresult to an
array, but I'd rather use a lazy solution.
My first steps would be to try
pragma(msg,
isInputRange!(typeof(groupedAes.front.map!((t)=>t.y))));
followed by
pragma(msg, isInputRange!(typeof(groupedAes.front)));
then
pragma(msg, typeof(groupedAes.front));
to see at what level it's going wrong.
