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