# \`Indexer\` and \`Int\` vs \`UInt\`
**URL:** https://forum.modular.com/t/indexer-and-int-vs-uint/2210
**Category:** Standard Library
**Created:** [September 2, 2025, 3:48am UTC](https://forum.modular.com/t/indexer-and-int-vs-uint/2210 "2025-09-02T03:48:21Z")
**Posts on this page:** 1
**Showing post:** 123
### Author:  [@Nick](https://forum.modular.com/u/Nick)
#### Post date: [September 13, 2025, 1:32am UTC](https://forum.modular.com/t/indexer-and-int-vs-uint/2210/123 "2025-09-13T01:32:13Z")
Here is a second proposal, which I think many people would prefer to [proposal A](https://forum.modular.com/t/indexer-and-int-vs-uint/2210/117).
## Proposal B: Keep `Int` as the default type, but take steps to ensure that people don’t misuse it.
Readers who are fans of Mojo’s current design will prefer this proposal, because only a few small changes need to be made.
As mentioned earlier, `Int` is misused whenever it stores a quantity unrelated to the number and/or position of values stored in memory. For example, you should never use an `Int` to store `site_visits` or `world_population` etc. By doing so, you are writing a program that is likely to break when deployed to certain targets.
### Mitigation 1: Rename `Int`, so that people understand its purpose
In today’s Mojo, programmers are likely to misuse `Int`, because:
- Its name is so simple (simpler than `Int64` etc), that it seems like the type you’re supposed to use whenever you want a “normal” integer. But in reality, you are only supposed to use to _count_ or _locate_ values in memory. If you use it for any other purpose, you will end up with code that is **not portable.**
- The name “int” has a different meaning in other language communities. In C++ it’s de facto 32 bits, and in Python it’s a BigInt. Overloading “int” with a third meaning is a recipe for confusion, especially for programmers who are adding Mojo into their existing Python and/or C++ projects.
I propose renaming `Int` to `Len`. Rationale:
- This name implies that `Len` is for storing lengths, sizes, and offsets into memory, especially given the implied connection to the `len` function.
- The name is reminiscent of `ssize_t` in C++ and `isize` in Rust.
- Intuitively, `var site_visits: Len` and `var button_clicks: Len` now feel like they have the wrong type, which is good, because they _do_!
As a bonus, `Len` ties in nicely with 0-based indexing. The array element `x[0]` is located at “length 0” from the start of the array.
### Mitigation 2: Generate a compiler warning whenever an integer literal ≥ 2^31 is converted to a `Len`.
Large `Len` literals usually indicate a programming error. The now-famous 🫢 `world_population` statement is one such example:
- `var world_population = 8142000000` **MISTAKE:** Has nothing to do with the size of memory.
Beyond being a programming mistake, this statement is also non-portable. On 32-bit platforms, it would either fail to compile, or worse—it would overflow.
The sensible thing to do here would be to emit a compiler warning saying “Warning: This statement will not compile on all targets. Large `Len` literals are usually a mistake. Did you mean to declare an `Int64`?”
### Mitigation 3: Prevent adding `Int64` to `Len` variables, and suggest that the `Len` variable should be an `Int64`.
This ties into the recent discussions about implicit casting. We should disallow code snippets like the following:
```mojo
var nums: List[Int64] = ...
var total = 0
for num in nums:
if :
total += nums
```
The problem with this program is that it is prone to overflow on 32-bit targets. As long as `Len. __iadd__ ` doesn’t accept an `Int64` on the RHS, and we don’t allow `Int64` to be implicitly converted to `Len`, we should be good here. However, if we want to really help our users out, the error message should suggest declaring `total` to be an `Int64`, since that is likely what the programmer meant to do!
### A decent compromise?
This is a modest proposal. Its goal is to ensure that Mojo users work with integers in a manner that is both correct and portable, while retaining the “Int by default” behaviour of today’s Mojo.
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