Question about: ("1.1").to!int;

[Jack]

On Wednesday, 21 October 2020 at 22:50:27 UTC, matheus wrote:
> Hi,
>
> import std.stdio, std.conv;
> void main(string[ ] args) {
>     auto a = (1).to!int;     // this works
>     auto b = ("1").to!int;   // this works
>     auto c = (1.1).to!int;   // this works and c = 1
>     auto d = ("1.1").to!int; // Doesn't work
> }
>
> The forth line gives me:
>
> std.conv.ConvException@/usr/include/dlang/dmd/std/conv.d(1898): 
> Unexpected '.' when converting from type string to type int
> ----------------
> ??:? pure @safe int std.conv.toImpl!(int, 
> immutable(char)[]).toImpl(immutable(char)[]) [0x55de76d9b4d7]
> ??:? pure @safe int 
> std.conv.to!(int).to!(immutable(char)[]).to(immutable(char)[]) 
> [0x55de76d99a17]
> ??:? _Dmain [0x55de76d9986e]
>
> Question:
>
> Since (1.1).to!int = 1, shouldn't the string value 
> ("1.1").to!int at least try to convert to float/double and then 
> to int?
>
> Because for what I see "to!int" converts from: int,real but in 
> case of string only when it is a integer representation?
>
> Matheus.

In first case you asked to convert a float (inferred by templace 
function) to int in the second case, you asked for the function 
"convert this string contaning a set of digits to an int for me". 
Doesn't make sense do something else than the asked. D has static 
if() and a nice way to do type comparasion at compile time. It 
does optimize things alot.

I haven't read the to source code, but consider it may use the 
idea like this:

auto to(T, T2)(T2 source)
{
     // you asked for int, doesn't makes sense consider a float 
here.
     // that would waste time checking for floats when you 
programmer's requested
     // was clear: an integer.
     static if(is(T == int) && is(T2 == string))
     {
         int v = 0;
         foreach(c; source)
             v = v * 10 + c - '0';
         return v;
     }
     // the string to float point conversion algorithm
     static if(is(T == float) && is(T2 == string))
     {
         /* ... */
     }
     static if(is(T == int) && is(T2 == double))
     {
         return cast(int)source;
     }
     assert(0, "unknow type");
}