f-strings

String interpolation for Lua, inspired by f-strings, a form of string interpolation coming in Python 3.6.

This module started as a blog post showcasing that Lua already has all features necessary to implement the equivalent Python feature.

Usage

Require the F module and then use it directly on strings. Any {expression} occurring in them will be evaluated and interpolated directly in the string.

local F = require("F")

local f = 99
local c = (f - 32) / 9 * 5
print(F"{f} degrees Fahrenheit is {c:%.2f} degrees Celsius")

Almost any Lua expression can be inserted (apart from two minor parsing caveats: ":%" is used as a separator for specifying formatting (according to the same rules of string.format), and curly braces within expressions must be well-nested).

See the examples directory for more examples.

Caveats

F uses Lua's debug library to obtain at runtime the values of the variables defined in the current scope. However, this information is not always available, in which case F might return unexpected results.

The first problematic situation is that F cannot see local variables when it is used in a tail-call position:

do
  local g = function(x) return F'hello {x}' end
  print(g("world")) -- prints 'hello nil' instead of 'hello world'
end

The second problematic setting is that F can't see variables in outer functions when those variables are not referenced in any inner functions:

do
  local x = "world"
  (function()
    print(F'hello {x}') -- prints 'hello nil' instead of 'hello world'
  end)()
end

One possible workaround is to pass the outer variable as an additional parameter to F. Although F ignores all arguments other than the template string, just using the outer variable is enough to cause Lua to package it into an upvalue, thus making it visible to F:

do
  local x = "world"
  (function()
    print(F('hello {x}', x))
  end)()
end

Author

Hisham Muhammad - http://twitter.com/hisham_hm - http://hisham.hm/

License

MIT/X11, the same as Lua.