[Issue 12654] New: std.typecons.ErrorMsg

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=12654

          Issue ID: 12654
           Summary: std.typecons.ErrorMsg
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: Phobos
          Assignee: nobody at puremagic.com
          Reporter: bearophile_hugs at eml.cc

I suggest to add to std.typecons a cons named "ErrorMsg" (or a similar name)
that is meant as a standard way to return an error condition when you don't
want to use exceptions (useful in nothrow code).

The advantage over Nullable is that it's not just "null" or a value. ErrorMsg
is a value or a error message, so it informs what the cause of the failure is.
To avoid generating many unused error message strings, I suggest the error
message to be not just a string, but a 0-arguments callable that returns a
string lazily.

The advantage over using a user defined struct or Algebraic is that this is
standard, so it gives a standard API, and the lazyness of the error message
allows to save computation time to generate the error string.

(A similar algebraic is used in other functional languages.)

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