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https://github.com/bkase/DoctorPretty
Wadler's "A prettier printer" embedded pretty-printer DSL for Swift
https://github.com/bkase/DoctorPretty
dsl functional ios prettier prettier-printer pretty-print swift
Last synced: about 2 months ago
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Wadler's "A prettier printer" embedded pretty-printer DSL for Swift
- Host: GitHub
- URL: https://github.com/bkase/DoctorPretty
- Owner: bkase
- License: apache-2.0
- Created: 2017-05-21T07:58:53.000Z (over 7 years ago)
- Default Branch: master
- Last Pushed: 2021-10-14T18:46:07.000Z (almost 3 years ago)
- Last Synced: 2024-07-21T15:09:26.561Z (2 months ago)
- Topics: dsl, functional, ios, prettier, prettier-printer, pretty-print, swift
- Language: Swift
- Size: 49.8 KB
- Stars: 192
- Watchers: 7
- Forks: 11
- Open Issues: 1
-
Metadata Files:
- Readme: README.md
- License: LICENSE
Awesome Lists containing this project
README
## Doctor Pretty
> A Swift implementation of the [A prettier printer (Wadler 2003)](https://homepages.inf.ed.ac.uk/wadler/papers/prettier/prettier.pdf) ported from [wl-pprint-annotated](https://github.com/minad/wl-pprint-annotated/blob/master/src/Text/PrettyPrint/Annotated/WL.hs)
[](https://travis-ci.org/bkase/DoctorPretty)
## What is this
A pretty printer is the dual of a parser -- take some arbitrary AST and output it to a String. This library is a collection of combinators and a primitive called a `Doc` to describe pretty printing some data much like a parser combinator library provides combinators and primitives to describe parsing test into an AST. Interestingly, this implementation efficiently finds the _best_ pretty print. You encode your knowledge of what the _best_ means with your use of various `Doc` combinators.
For example: Let's say we have some internal structured representation of this Swift code:
```swift
func aLongFunction(foo: String, bar: Int, baz: Long) -> (String, Int, Long) {
sideEffect()
return (foo, bar, baz)
}
```
With this library the description that pretty prints the above at a page width of 120 characters. Also prints:
At a page-width of 40 characters:
```swift
func aLongFunction(
foo: String, bar: Int, baz: Long
) -> (String, Int, Long) {
sideEffect()
return (foo, bar, baz)
}
```
and at a page-width of 20 characters:
```swift
func aLongFunction(
foo: String,
bar: Int,
baz: Long
) -> (
String,
Int,
Long
) {
sideEffect()
return (
foo,
bar,
baz
)
}
```
See the encoding of this particular document in the [`testSwiftExample` test case](Tests/DoctorPrettyTests/DoctorPrettyTests.swift).
## What would I use this for?
If you're outputting text and you care about the width of the page. Serializing to a `Doc` lets you capture your line-break logic and how your output string looks in one go.
Why would you output text and care about page width?
1. You're building a `gofmt`-type tool for Swift (Note: `gofmt` doesn't pretty-print based on a width, `refmt` (Reason) and `prettier` (JavaScript) do)
2. You're writing some sort of codegen tool to output Swift code
3. You're building a source-to-source transpiler
2. You're outputing help messages in a terminal window for some commandline app (I'm planning to use this for https://github.com/bkase/swift-optparse-applicative)
## What is this, actually
A Swift implementation of the [A prettier printer (Wadler 2003)](https://homepages.inf.ed.ac.uk/wadler/papers/prettier/prettier.pdf) paper (including generally accepted modern enhancements ala [wl-pprint-annotated](https://github.com/minad/wl-pprint-annotated/blob/master/src/Text/PrettyPrint/Annotated/WL.hs). This implementation is close to a direct port of [wl-pprint-annotated](https://github.com/minad/wl-pprint-annotated/blob/master/src/Text/PrettyPrint/Annotated/WL.hs) with some influence from [scala-optparse-applicative's Doc](https://github.com/bmjames/scala-optparse-applicative/blob/master/src/main/scala/net/bmjames/opts/types/Doc.scala) and a few extra Swiftisms.
## Basic Usage
`Doc` is very composable. First of all it's a [`monoid`](https://www.youtube.com/watch?v=6z9QjDUKkCs) with an `.empty` document and the `.concat` case which just puts two documents next to each other. We also have a primitive called `grouped`, which tries this document on a single line, but if it doesn't fit then breaks it up on new-lines. From there we build all high-level combinators up.
`x <%> y` concats x and y with a space in between if it fits, otherwise puts a line.
```swift
.text("foo") <%> .text("bar")
```
pretty-prints under a large page-width:
```
foo bar
```
but when the page-width is set to `5` prints:
```
foo
bar
```
Here are a few more combinators:
```swift
/// Concats x and y with a space in between
static func <+>(x: Doc, y: Doc) -> Doc
/// Behaves like `space` if the output fits the page
/// Otherwise it behaves like line
static var softline
/// Concats x and y together if it fits
/// Otherwise puts a line in between
static func <%%>(x: Doc, y: Doc) -> Doc
/// Behaves like `zero` if the output fits the page
/// Otherwise it behaves like line
static var softbreak: Doc
/// Puts a line between x and y that can be flattened to a space
static func <&>(x: Doc, y: Doc) -> Doc
/// Puts a line between x and y that can be flattened with no space
static func <&&>(x: Doc, y: Doc) -> Doc
```
There are also combinators for turning collections of documents into "collection"-like pretty-printed primitives such as a square-bracket separated lists:
```swift
.text("let x =") <%> [ "foo", "bar", "baz" ].map(Doc.text).list(indent: 4)
```
Pretty-prints at page-width 80 to:
```swift
let x = [foo, bar, baz]
```
and at page-width 10 to:
```swift
let x = [
foo,
bar,
baz
]
```
See the source for more documentation, I have included descriptive doc-comments to explain all the operators (mostly taken from [wl-pprint-annotated](https://github.com/minad/wl-pprint-annotated/blob/master/src/Text/PrettyPrint/Annotated/WL.hs)).
## How do I actually pretty print my documents?
`Doc` has two rendering methods for now: `renderPrettyDefault` prints with a page-width of 100 and `renderPretty` lets you control the page-width.
These methods don't return `String`s directly -- they return `SimpleDoc` a low-level IR that is close to a string, but high-enough that you can efficiently output to some other output system like stdout or a file.
For now, `SimpleDoc` has `displayString()` which outputs a `String`, and:
```swift
func display(readString: (String) -> M) -> M
```
`display` takes a function that can turn a string into a monoid and then smashes everything together. Because this works for any monoid, you just need to provide a monoid instance for your output formatter (to write to stdout or to a file).
## Installation
With Swift Package Manager, put this inside your `Package.swift`:
```swift
.Package(url: "https://github.com/bkase/DoctorPretty.git",
majorVersion: 0, minor: 5)
```
## How does it work?
Read the [A prettier printer (Wadler 2003) paper](https://homepages.inf.ed.ac.uk/wadler/papers/prettier/prettier.pdf).
`Doc` is a recursive enum that captures text and new lines. The interesting case is `.union(longerLines: Doc, shorterLines: Doc)`. This case reifies the notion of "try the longer lines first, then the shorter lines". We can build all sorts of high-level combinators that combine `Doc`s in different ways that eventually reduce to a few strategic `.union`s.
The renderer keeps a work-list and each rule removes or adds pieces of work to the list and recurses until the list is empty. The best-fit metric proceeds greedily for now, but can be swapped out easily for a more intelligent algorithm in the future.