On 2004-05-17 at 09:43BST Graham Klyne wrote:
I guess that extensible syntax is great for language experiments, but I'd be concerned about it becoming a common feature of production programming.
I (the design of Ponder notwithstanding) think that too.
I have found that even the simple facility of user-defined operators (not including the `name` form) leads to code that is far harder to pick up and understand.
That's probably true. I've often wondered if there was a way of restricting parts of the language to use within widely available libraries (as opposed to user-defined modules). For a user to add an instance of a class, and so change the meaning of '+' is not unreasonable, but more than slight use of weird combinations of symbols surely increases the overhead when reading code.
I hypothesize (based on my own experience) that when picking up some new code, a programmer relies to a considerable extent on the mnemonic significance of the names used in the code. Non-mnemonic elements of the code must be learned and committed to memory before they can be read with any facility. This suggests that user-defined syntax (and operators) run the risk of making the task of understanding a new program more akin to learning a new programing language.
I concur. My desire to have a language with extensible syntax derives from a desire to impose some sort of consistency on additions to the language, which makes them easier to absorb when they do arrive, but it's tempered with a wish to limit the use of such facilities to places where it's really necessary. The addition of regular expressions might well be a suitable case for treatment and an argument that the current extension facilities aren't quite enough, but I wouldn't want to see too general a mechanism for the reasons you give. Jón -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk