The version of readln you are using is[0]. This works by taking in a
buffer of memory to write out, and returns how many codepoints were stored.
Because you are not reusing memory, not using this form you can of
course use string[1] instead, rather than ``char[]``.
```d
string s = readln();
```
The definition of string is[2]:
```d
alias string = immutable(char)[];
```
Note the immutable there, which means that each value in the slice
cannot be modified. Hence why it can't be used as a buffer.
[0] https://dlang.org/phobos/std_stdio.html#.readln.2
[1] https://dlang.org/phobos/std_stdio.html#.readln
[2] https://github.com/dlang/dmd/blob/master/druntime/src/object.d#L69
