gap

A Gleam library for comparing strings/lists and producing a textual (styled) representation of the differences.

A typical styled output from the comparison can look like this:

Installation

If available on Hex this package can be added to your Gleam project:

gleam add gap

Documentation can be found at https://hexdocs.pm/gap.

Usage

Introduction

Gap implements string/list comparison by finding the longest common subsequence. The result of the comparison are two sequences (one for each of the compared strings/lists) consisting of subsequences that are annotated as matching or non-matching.

For example comparing the strings in the example above will look as follows:

let comparison =
compare_strings(
   "lucy in the sky with diamonds",
   "lucy is  the shy with diagrams",
)
|> io.debug()
// StringComparison(
//   [
//     Match(["l", "u", "c", "y", " ", "i"]),
//     NoMatch(["n"]),
//     Match([" ", "t", "h", "e", " ", "s"]),
//     NoMatch(["k"]),
//     Match(["y", " ", "w", "i", "t", "h", " ", "d", "i", "a", "m"]),
//     NoMatch(["o", "n", "d"]),
//     Match(["s"]),
//   ],
//   [
//     Match(["l", "u", "c", "y", " ", "i"]),
//     NoMatch(["s", " "]),
//     Match([" ", "t", "h", "e", " ", "s"]),
//     NoMatch(["h"]),
//     Match(["y", " ", "w", "i", "t", "h", " ", "d", "i"]),
//     NoMatch(["a", "g", "r"]),
//     Match(["a", "m", "s"]),
//   ],
// )

Styling

This is useful information but a bit overwhelming to look at (specially for longer string) so the library has some built in functions to display the differences using colors instead.

Using the same example again we can style the result and print it to the console

let comparison =
compare_strings(
   "lucy in the sky with diamonds",
   "lucy is  the shy with diagrams",
)
|> to_styled()
io.println(comparison.first)
io.println(comparison.second)

This will give us something similar to the output above.

Comparing list

It is also possible to compare lists with elements of arbitrary types.

pub type Warning {
 Please
 Mind
 The(what: String)
}

compare_lists([Mind, The("Gap")], [Please, Mind, The("What")])
|> io.debug()
// ListComparison(
//   [Match([Mind]), NoMatch([The("Gap")])],
//   [NoMatch([Please]), Match([Mind]), NoMatch([The("What")])],
// )

Customize styling

The visual representation of the comparison can be customized. To do this use a Styling created from the comparison that should be styled. This example uses gleam_community/ansi to highlight the non-matches in different colors.

let comparison =
compare_strings(
   "Strings are made of smaller things",
   "Things are maybe smaller string",
)
|> from_comparison()
|> highlight(
   fn(first) { ansi.cyan(first) },
   fn(second) { ansi.magenta(second) },
   fn(matching) { matching },
)
|> to_styled_comparison()
io.println(comparison.first)
io.println(comparison.second)

This will output something similar to this

Serialization

Furthermore it is also possible to customize the styling by changing the way that the comparison is serialized. An easy way to do this is to use the utility function mk_generic_serializer which creates a serializer which some specific separator and a hook for surrounding the result with some content. Here is a somewhat contrived example

let comparison =
compare_lists(["one", "two", "three"], ["two", "two", "tree"])
|> from_comparison()
|> highlight(
   fn(first) { first <> " was not found in other" },
   fn(second) { second <> " was not found in other" },
)
|> serialize(mk_generic_serializer(
   ", and ",
   fn(result) { "Comparing the lists gave the following result: " <> result },
))
|> to_styled_comparison()
io.println(comparison.first)
io.println(comparison.second)
// Comparing the lists gave the following result: "one" was not found in other, and "two" was found in other, and "three" was not found in other
// Comparing the lists gave the following result: "two" was not found in other, and "two" was found in other, and "tree" was not found in other

Custom serialization

Serializers can of course have a custom implementation. The following example utilizes this together with custom highlighters, creating a patch-like output (this is not exactly the same as patch-format since that shows the changes in relation to the original - to do that both lists of matching/non-matching lines would need to be processed together)

let comparison =
compare_lists(
   [
   "pub type Gap = List(EmptyString)", "", "pub type Traveler {",
   "  OnTrain", "  OverGap(gap: Gap)", "  OnPlatform", "}",
   ],
   [
   "pub type Traveler {", "  OnTrain", "  OverGap(gap: String)",
   "  OnPlatform", "}",
   ],
)
|> from_comparison()
|> highlight(
   fn(first) { "+" <> first },
   fn(second) { "-" <> second },
   fn(matching) { " " <> matching },
)
|> serialize(fn(part) {
   case part {
   Part(acc, lines, highlight) ->
       acc <> {
       lines
       |> list.map(fn(line) { highlight(line) })
       |> string.join("\n")
       } <> "\n"
   All(result) -> result
   }
})
|> to_styled_comparison()
io.println(comparison.first)
io.println(comparison.second)
// +pub type Gap = List(EmptyString)
// +
//  pub type Traveler {
//    OnTrain
// +  OverGap(gap: Gap)
//    OnPlatform
//  }
// 
//  pub type Traveler {
//    OnTrain
// -  OverGap(gap: String)
//    OnPlatform
//  }