playgroundliterals.md

Modernizing Playground Literals

• Proposal: TBD
• Author(s): Erica Sadun
• Status: TBD
• Review manager: TBD

Introduction

Playground literals tokenize colors, files, and images. They provide drag-and-drop convenience and in-context visualizations that offer easy reference and manipulation when designing playground content. These literals are built using a simple square bracket syntax that, in the current form, conflicts with collection literals. This proposal redesigns playground literals to follow the precedent of #available and #selector.

Discussion took place on the Swift Evolution mailing list in the [Discussion] Modernizing Playground Literals thread. Thanks to Chris Lattner for suggesting this enhancement.

Motivation

Color, image, and file literals are currently represented as:

[#Color(colorLiteralRed: red, green: green, blue: blue, alpha: alpha)#]
[#Image(imageLiteral: localResourceNameAsString)#]
[#FileReference(fileReferenceLiteral: localResourceNameAsString)#]

Playground literals present the following features:

• They appear within a container designated with [# #] endpoints.
• They are marked with a capital camel case role name.
• First label arguments use the word literal to punch the construction of each literal item.

There are several issues with this approach:

• The enclosing square brackets conflict with collection literals, adding extra work for parsing.
• The construction syntax does not follow modern Swift conventions.
• The word literal describes the constructed items not the argument being passed to create the literal. It is misplaced in its current use.

Detail Design

Simplifying constructors to octothorpe-delineated identifiers cleans up the language, removes potential grammar conflicts, and follows precedent for other identifiers used in modern Swift. Our proposed identifiers are #colorliteral, #imageliteral, and #fileliteral.

color-literal → #colorliteral(red: unit-floating-point-literal, green: unit-floating-point-literal, blue: unit-floating-point-literal, alpha: unit-floating-point-literal)
unit-floating-point-literal → floating point number greater or equal to zero, less than or equal to one

image-literal → #imageliteral(imageName: image-resource-name)
image-resource-name → static-string-literal referring to image resource name

file-literal → #fileliteral(resourceName: file-resource-name)
file-resource-name → static-string-literal referring to local resource name

In this design:

• Each redesigned identifier uses lower case, to match existing Swift literals.
• Arguments use lower camel case labels, as is conventional.
• The word literal is added to identifiers denoting each item's role.
• The arguments are simplified and standardized to red, green, blue, alpha, imageName, and resourceName.

Alternatives Considered

#resourceliteral may better describe a file resource than #fileliteral.