rfcs/docs/pretty-tables.md

table.pretty and changes to printing tables

Summary

We will add a function to display a pretty version of a table for debugging purposes. We will use this function as the backbone to changing the default implementation of print(table) in CLI contexts.

Motivation

In Roblox Studio, printing out a table with "Log mode" off will display an interactive table. In roblox-cli, Lune, and luau.exe, it will print the memory address, which is faintly useful, and makes debugging tables a massive chore. Making tables pretty print is a clear win in this regard.

Adding a function to create these pretty forms and using that as the underlying implementation behind print is useful for custom loggers that do not use print and go to file, or for something like debug commands.

Design

table.pretty

The following function will be added to the standard library:

table.pretty(input: { [any]: any }, indentation: number?): string

indentation defaults to 2.

For a non-empty table, this will perform the following behavior:

1. The initial opening and ending brace will be on their own line
2. Form the string [$key]: $value, for every key and value, based on the same behavior as generalized iteration, even using __iter if it exists.
   • $key and $value will be pretty printed if it is both a table and if it would be acceptable to do so (no __metatable). This will be indented indentation number of spaces.
   • If $value is a string, it will be wrapped in quotes.
   • Otherwise, $key and $value will be the result of tostring(value).
   • The comma is always included.
   • $key is not included if it is a number part of a sequence of keys starting with 1.
3. In the event of a cyclic table with no tostring implementation, the literal string "<cyclic: memory address>" will be put in its place.

If indentation is 0, then it will all be put onto a single line, with spaces being used in place of new lines. In this case, trailing commas will not be present.

Examples

print(table.pretty({
	width = 12,
	toppings = {
		"pepperoni",
		"sausage",
	},
}))

-->

{
  width = 12,
  toppings = {
    "pepperoni",
    "sausage",
  },
}

---

print(table.pretty({
  "my cool",
  "mixed table",

  key = "value",
  [150] = "awesome",
}))

-->

{
  "my cool",
  "mixed table",
  key = "value",
  [150] = "awesome",
}

---

print(table.pretty({
  "my",
  "table",
  {
    x = 1,
    y = 2,
  }
}, 0))

-->

{ "my", "table", { x = 1, y = 2 } }

---

local t = { x = 1 }
t.y = t
print(table.pretty(t))

-->

{
  x = 1,
  y = <cyclic: 0xDEADBEEF>,
}

Caveats

An empty table will always produce "{}".

Calling table.pretty on an input table with a custom __metatable will throw an error.

Changes to printing tables

The default behavior of printing will change from calling __tostring to instead using __tostring if it exists, and table.pretty(t) if it does not. The existing behavior of printing a memory address will still be available through print(tostring(t)), which is already useful in a context like Roblox when working with Log mode.

Roblox

While Luau is not Roblox specific, it is nevertheless useful to clarify the expected behavior in that environment. Roblox Studio is expected to work the same way as it does now when "Log mode" is disabled--an interactive table expansion. When "Log mode" is enabled, the new printing behavior will take effect.

Drawbacks

TODO:

• LogService breaking change

Alternatives

TODO:

• implement one and not the other
• use tabs
• more configuration like spaces/tabs/column width (Python pprint)
• implement nothing, only change in roblox-cli
• custom metamethod for printing/pretty
• no trailing comma