mirror of
https://github.com/ProvableHQ/leo.git
synced 2024-12-23 10:12:21 +03:00
120 lines
3.8 KiB
Markdown
120 lines
3.8 KiB
Markdown
# Leo RFC 013: Constant Functions
|
|
|
|
## Author(s)
|
|
|
|
The Aleo Team.
|
|
|
|
## Status
|
|
|
|
IMPLEMENTED
|
|
|
|
## Summary
|
|
|
|
This RFC proposes the additional of an optional `const` modifier to function declarations
|
|
to explicitly designate functions that return constant values that can be calculated at compile time.
|
|
|
|
## Motivation
|
|
|
|
Explicitly designating constant functions makes user intention explicit
|
|
and simplifies the Leo compiler's checks, as explained below.
|
|
|
|
## Background
|
|
|
|
Leo code is partially evaluated on its `const` inputs prior to being translated to R1CS.
|
|
A function that returns a value that only depends on `const` inputs directly or indirectly,
|
|
can be partially evaluated away, without having to be inlined during flattening.
|
|
|
|
## Design
|
|
|
|
### Syntax
|
|
|
|
The ABNF grammar is extended by adding an optional `const` modifier to function declarations:
|
|
```
|
|
function-declaration = *annotation [ %s"const" ] %s"function" identifier
|
|
"(" [ function-parameters ] ")" [ "->" type ]
|
|
block
|
|
```
|
|
|
|
This applies to both top-level and member functions.
|
|
|
|
### Static Semantics
|
|
|
|
A `const` function declaration must satisfy the following conditions:
|
|
* All its parameters are `const`, including the `self` parameter for instance circuit member functions.
|
|
* The body does not reference the special `input` variable.
|
|
* The body only calls other `const` functions.
|
|
|
|
### Dynamic Semantics
|
|
|
|
This has no impact on the dynamic semantics of Leo, viewed as a traditional programming language.
|
|
|
|
### Flattening
|
|
|
|
Given that `const` expressions are evaluated completely during flattening,
|
|
the values of the arguments of a `const` function call are known during flattening,
|
|
and therefore the function call can be completely evaluated as well.
|
|
|
|
If the function is recursive (individually or with others),
|
|
the evaluation involves the bounded recursion analysis described in a separate RFC.
|
|
|
|
### Implementation Considerations
|
|
|
|
ASTs for function declarations are extended with a boolean flag `const_`.
|
|
|
|
If a `const` function has a non-`const` parameter,
|
|
an ASG error occurs.
|
|
|
|
If the body of a `const` function references the `input` variable or calls a non-`const` function,
|
|
an ASG error occurs.
|
|
|
|
The description of static semantics, dynamic semantics, and flattening given above
|
|
are expressed in terms of Leo, because that is the user's view of the language.
|
|
In the implementation, flattening occurs after the Leo code is translated to the IR.
|
|
|
|
### Examples
|
|
|
|
```ts
|
|
const function len(const arr: [u8; _]) -> u32 {
|
|
return arr.len();
|
|
}
|
|
|
|
circuit Sample {
|
|
x: [char; 5]
|
|
const function say_hi(const self) -> [char; 5] {
|
|
return self.x;
|
|
}
|
|
}
|
|
```
|
|
|
|
## Drawbacks
|
|
|
|
This extension does not appear to bring any drawbacks.
|
|
|
|
## Effect on Ecosystem
|
|
|
|
None.
|
|
|
|
## Alternatives
|
|
|
|
### No Constant Designation
|
|
|
|
Without an explicit designation of constant functions,
|
|
the Leo compiler needs to perform an inter-procedural analysis:
|
|
if `f` calls `g`, in order for `f` to be constant, also `g` must be constant.
|
|
In other words, the call graph must be taken into account.
|
|
|
|
In contrast, with the `const` designation,
|
|
an intra-procedural analysis suffices,
|
|
as discussed in the static semantics section above.
|
|
|
|
## Future Extensions
|
|
|
|
In other languages like Rust, `const` functions are not required to have all constant parameters.
|
|
They are just required to return constant results for constant arguments,
|
|
i.e. they must not access global variables and they must only call other `const` functions.
|
|
In other words, these `const` functions are polymorphic over "constancy".
|
|
|
|
This could be also realized in Leo, because type inference/checking determines `const` and non-`const` expressions.
|
|
This tells the compiler which function calls have all `const` arguments and which ones do not.
|
|
Therefore, the compiler can selectively evaluate, during flattening, only the calls of `const` functions on `const` arguments.
|