Use variables inside a string in lua - lua

Question

In javascript we can do the following:
var someString = `some ${myVar} string`
I have the following lua code, myVar is a number that needs to be in the square brackets:
splash:evaljs('document.querySelectorAll("a[title*=further]")[myVar]')

Answer 1

Function that fits you description is string.format:
splash:evaljs(string.format('document.querySelectorAll("a[title*=further]")[%s]', myVar))
It is not as verbose as ${}. It is more of a good old (and hated) sprintf.

Answer 2

I have written an emulation of Python's f'' strings, which is a set of functions you can hide inside a require file. So, if you like Python's f'' strings, this may be what you're looking for.
(If anyone finds errors, please notify.)
It's quite big compared to the other solution, but if you hide the bulk in a library, then its use is more compact and readable, IMO.
With this library you can do the following, for example:
require 'f_strings'
a = 12345
print(f'Number: {a}, formatted with two decimals: {a::%.2f}')
-- Number: 12345, formatted with two decimals: 12345.00
Note the use of Lua string.format formatting codes, and the use of double colon (instead of Python's single colon) for format specifiers because of Lua's use of colon for methods.
I have extracted only the relevant functions from a larger library. Although some optimizations may be possible for this specific use case, I leave them unchanged as they are general purpose and may also be useful for other purposes.
And here's the required library (placed somewhere in your Lua libraries folder):
-- f_strings.lua ---
unpack = table.unpack or unpack
--------------------------------------------------------------------------------
-- Escape special pattern characters in string to be treated as simple characters
--------------------------------------------------------------------------------
local
function escape_magic(s)
local MAGIC_CHARS_SET = '[()%%.[^$%]*+%-?]'
if s == nil then return end
return (s:gsub(MAGIC_CHARS_SET,'%%%1'))
end
--------------------------------------------------------------------------------
-- Returns iterator to split string on given delimiter (multi-space by default)
--------------------------------------------------------------------------------
function string:gsplit(delimiter)
if delimiter == nil then return self:gmatch '%S+' end --default delimiter is any number of spaces
if delimiter == '' then return self:gmatch '.' end
if type(delimiter) == 'number' then --break string in equal-size chunks
local index = 1
local ans
return function()
ans = self:sub(index,index+delimiter-1)
if ans ~= '' then
index = index + delimiter
return ans
end
end
end
if self:sub(-#delimiter) ~= delimiter then self = self .. delimiter end
return self:gmatch('(.-)'..escape_magic(delimiter))
end
--------------------------------------------------------------------------------
-- Split a string on the given delimiter (comma by default)
--------------------------------------------------------------------------------
function string:split(delimiter,tabled)
tabled = tabled or false --default is unpacked
local ans = {}
for item in self:gsplit(delimiter) do
ans[#ans+1] = item
end
if tabled then return ans end
return unpack(ans)
end
--------------------------------------------------------------------------------
function copy(t) --returns a simple (shallow) copy of the table
if type(t) == 'table' then
local ans = {}
for k,v in next,t do ans[ k ] = v end
return ans
end
return t
end
--------------------------------------------------------------------------------
function eval(expr,vars)
--evaluate a string expression with optional variables
if expr == nil then return end
vars = vars or {}
assert(type(expr) == 'string','String expected as 1st arg')
assert(type(vars) == 'table','Variable table expected as 2nd arg')
local env = {abs=math.abs,acos=math.acos,asin=math.asin,atan=math.atan,
atan2=math.atan2,ceil=math.ceil,cos=math.cos,cosh=math.cosh,
deg=math.deg,exp=math.exp,floor=math.floor,fmod=math.fmod,
frexp=math.frexp,huge=math.huge,ldexp=math.ldexp,log=math.log,
max=math.max,min=math.min,modf=math.modf,pi=math.pi,pow=math.pow,
rad=math.rad,random=math.random,randomseed=math.randomseed,
sin=math.sin,sinh=math.sinh,sqrt=math.sqrt,tan=math.tan,
tanh=math.tanh}
for name,value in pairs(vars) do env[name] = value end
local a,b = pcall(load('return '..expr,nil,'t',env))
if a == false then return nil,b else return b end
end
--------------------------------------------------------------------------------
-- f'' formatted strings like those introduced in Python v3.6
-- However, you must use Lua style format modifiers as with string.format()
--------------------------------------------------------------------------------
function f(s)
local env = copy(_ENV) --start with all globals
local i,k,v,fmt = 0
repeat
i = i + 1
k,v = debug.getlocal(2,i) --two levels up (1 level is this repeat block)
if k ~= nil then env[k] = v end
until k == nil
local
function go(s)
local fmt
s,fmt = s:sub(2,-2):split('::')
if s:match '%b{}' then s = (s:gsub('%b{}',go)) end
s = eval(s,env)
if fmt ~= nil then
if fmt:match '%b{}' then fmt = eval(fmt:sub(2,-2),env) end
s = fmt:format(s)
end
return s
end
return (s:gsub('%b{}',go))
end