applay for template function with sumtypes result?

[Steven Schveighoffer]

On 7/4/23 12:58 AM, kiriakov wrote:
> Hi. I can't handle template. It all looks sad, build ok, test error.
> 
> ```
> struct ParsResult(T) { T[] _in; T[] _out; }
> struct Err(T) { T[] x; }
> 
> struct Result(T) {
>      SumType!(ParsResult!T, Err!T) data;
>      alias data this;
>      this(Value)(Value value) { data = value; }
> }
> 
> 
> Result!T applay(T)(Result!T function (T[]) pure fun, T[] x) pure { 
> return fun(x); }
> ParsResult!char f (char[] x) pure { return ParsResult!char(x[1..$], 
> x[2..$]); }
> 
> unittest
> {
>      writeln(applay(&f,"String10".dup));
> }
> ```
> 
> I got
> 
> Error: none of the overloads of template `mexception.pars` are callable 
> using argument types `!()(ParsResult!char delegate(char[] x) pure 
> nothrow @nogc @safe, char[])`
> source/mexception.d(64,10):        Candidate is: `pars(T)(Result!T 
> function(T[]) pure fun, T[] x)`
> 

If inference isn't working you can explicitly instantiate the template 
to see why it isn't working.

After adding the necessary imports, I changed your unittest to:

```d
writeln(applay!char(&f,"String10".dup));
```

And I get the errors:

```
onlineapp.d(18): Error: function 
`onlineapp.applay!char.applay(Result!char function(char[]) pure fun, 
char[] x)` is not callable using argument types `(ParsResult!char 
function(char[] x) pure, char[])`
onlineapp.d(18):        cannot pass argument `& f` of type 
`ParsResult!char function(char[] x) pure` to parameter `Result!char 
function(char[]) pure fun`
```

It looks like a type mismatch. The function accepted should return 
`Result` but it's returning `ParsResult`.

-Steve