Defaulted Element Reference Syntax¶

DEP #:	10
Type:	Standards Track
Author:	Peter S. Housel
Status:	Draft
Created:	13-Oct-2018
Last-Modified:	24-Feb-2019
Post-History:	04-Mar-2019
Target-Version:	2019.1

Revision History¶

The revision history of this document is available here: https://github.com/dylan-lang/opendylan/commits/master/documentation/source/proposals/dep-0010-element-otherwise.rst

Abstract¶

The Dylan language currently provides bracketed element reference syntax as a shorthand for calls to the element and aref functions. This DEP proposes an element reference syntax for providing default values for element calls.

Motivation¶

The element reference syntax collection[i] is a compact way of writing element(collection, i). However, whenever supplying a default becomes necessary, element reference syntax cannot be used. Sometimes this results in a mixing of the two styles in the same area of code. Adding defaulted element reference syntax can improve the conciseness and readability of code.

Specification¶

To the operand nonterminal identifier we add the following phrase grammar rule:

operand: operand [ arguments _opt ] otherwise otherwise-operand

In addition we add the following nonterminal and phrase grammar rules:

otherwise-operand:

leaf

SYMBOL

Just as collection[i] is syntactic sugar for element(collection, i) under the interpretation of element in force for the module where it appears, the collection[i] otherwise y syntax is equivalent to element(collection, i, default: y).

Rationale¶

Note that to prevent parsing ambiguities the operand of the otherwise clause uses the leaf grammar nonterminal rather than the expression nonterminal used for function call arguments. This requires that complex default value expressions be parenthesized, but since in practice most supplied defaults are either literals or (constant) variable names this should not provide much of an inconvenience.

Backwards Compatibility¶

The addition of this new legal syntax does not affect the interpretation of existing programs.

Reference Implementation¶

A reference implementation for Open Dylan can be found at https://github.com/dylan-lang/opendylan/tree/element-otherwise