subst vs substr

So what is this thing about? As discussed mainly here and here there is a possible clash between two different methods in Perl 6 that have been abbreviated in a very similar way:
  • subst, short for substitute that performs a s// against a Str;
  • substr, short for substring, that extract data from a Str.
The latter, substr is a routine, that is a method that can be called as an operator against an object:
substr 'hello', 0, 1; # h
'hello'.substr: 0, 1; # h
The former, subst is a method, i.e., it is not possible to call it outside of an object instance:
> subst 'hello', 'h', 'H';
===SORRY!=== Error while compiling:
Undeclared routine:
    subst used at line 1. Did you mean 'substr'?

> 'hello'.subst: 'h', 'H';
# Hello
The problem, clearly, arises when you try to use any of the above as methods, that is against an object instance:
# mispelled 'substr' instead of 'subst'
> 'hello'.substr: 'h', 'H';
Earlier failure:
 Cannot convert string to number: base-10 number must begin with valid digits or '.' in '⏏h' (indicated by )
  in block <unit> at <unknown file> line 1

Final error:
 Type check failed in assignment to $from; expected Int but got Failure (
  in block <unit> at <unknown file> line 1

# mispelled 'subst'  instead of 'substr'
> 'hello'.subst: 0, 1;
Since the two methods differ only on the final ‘r’, it is quite easy to mispell them and got a compilation error or a wrong result. In fact, while substr cannot accept strings as input (unless they are number stringitied!), it will cause a compilation error. On the other side, ~subst does accept numbers as input and in the better case it does nothing (not finding any occurence of the first number). You get the point: the situation can quickly become a mess! That’s why there’s some thinking effort about if, how and when to rename any of the method to another name and, possibly, deprecate the old name. My personal idea, as I wrote in the discussion comments is to:
  • avoid deprecating any method, since they have been potentially used in many places so far;
  • provide a long method name for both keeping the short names in place, so that every developer can choose the style he likes the most.
Being substr a routine, particular emphasis should be kept to that, and therefore I strongly recommend to not change at all its name. On the other hand, being subst a method, it is possible to alias it and suggest developers to use the right name to avoid bugs due to the usge of the wrong method. It is not a simple task, and it surely opens a path for other similar cases to be managed in the future, and therefore it does require a particular care. Is this a possible API mistake in the Perl 6 language? I’m not so sure, but it is a surely possible annoying capability.

The article Perl 6 subst vs substr has been posted by Luca Ferrari on January 9, 2018

Tags: perl6