Compile Lua without automatic conversion between strings and numbers

Question

Lua is generally a strongly-typed language, providing almost no implicit conversion between data types.

However, numbers and strings do get automatically coerced in a few cases:

Lua provides automatic conversion between string and number values at run time. Any arithmetic operation applied to a string tries to convert this string to a number, following the rules of the Lua lexer. (The string may have leading and trailing spaces and a sign.) Conversely, whenever a number is used where a string is expected, the number is converted to a string, in a reasonable format

Thus:

local x,y,z = "3","8","11"
print(x+y,z)  --> 11   11
print(x+y==z) --> false
print(x>z)    --> true

I do not want this. How can I recompile the Lua interpreter to remove all automatic conversion?

I would prefer to have:

print(x+y)    --> error: attempt to perform arithmetic on a string value
print(x>1)    --> error: attempt to compare number with string
print(x..1)   --> error: attempt to concatenate a number value

Urumchi

Doable, but not out of the box. You'll have to edit the innards of Lua. Start at lua.org/source/5.2/lvm.c.html#luaV_tonumber. – Dishpan

You might want to "fix" cases like this, too: print("one="..1) – Cloudburst

Is creating your own dialect of lua, subtly (and silently!) different from mainstream really worth it? – Underlayer

@Underlayer Well, we've already compiled with 32-bit numbers instead of 64-bit, which has its own subtle implications (e.g. os.time() does not increase every second). That was supported by compile-time flags, though. – Urumchi

Can you please explicitly state which output you'd want (for your respective examples)? – Urata

@Urata Edited to add the desired output. – Urumchi

Answer 1

The illustrious LHF has commented above that this is not possible out of the box, and requires editing the innards of Lua, starting with http://www.lua.org/source/5.2/lvm.c.html#luaV_tonumber

Marking this as the answer in order to close this question. If anyone later chooses to provide an answer with in-depth details on what needs to be done, I will gladly switch the acceptance mark to that answer.

Urumchi

Agree, parameters get evaluated before being passed to the function, in this case "print". Hence the inner functioning of expression evaluation (IE coercion) needs to be changed. – Urata