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Modernizing Playground Literals

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(resourceName: 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, and resourceName.

Alternatives Considered

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