[Feature]: decode a slice of `FieldElement` into a list of `Token`

[haroune-mohammedi]

This PR addresses issue #286

We implemented a decode fucntion

fn decode(types: &[ParamType], data: &[FieldElement]) -> Result<Vec<Token>, DecoderError>

Where:

pub enum ParamType {
    FieldElement,
    Array,
    Tuple(usize),
}

#[derive(PartialEq, Eq, Debug)]
pub enum Token {
    FieldElement(FieldElement),
    Array(Vec<FieldElement>),
    Tuple(Vec<FieldElement>),
}

[fracek]

I like where you're going with this PR. My suggestion is to add a new trait for encoding and decoding Starknet types.

pub trait AbiEncode {
  /// Encode type to a token.
  // TODO: should it consume the type or should take a reference?
  fn encode(self) -> Token;
}

pub trait AbiDecode {
  type Error;
  
  fn decode(tokens: impl AsRef<[Token]>) -> Result<Self, Self::Error>;
}

Then it's possible to implement the encoder and decoder for all rust builtin types like numbers (u64, u32), and structs (vectors, tuples, and arrays).

[haroune-mohammedi]

@xJonathanLEI can you please take a look here and tell us what do you think ?

I added a macro that auto generates a Decode implementation for structs from a slice of Token, check https://github.com/xJonathanLEI/starknet-rs/blob/59f42eaddab7651e94474cd70f8872b27eadebea/examples/decoder_example.rs

[haroune-mohammedi]

I like where you're going with this PR. My suggestion is to add a new trait for encoding and decoding Starknet types.

pub trait AbiEncode {
  /// Encode type to a token.
  // TODO: should it consume the type or should take a reference?
  fn encode(self) -> Token;
}

pub trait AbiDecode {
  type Error;
  
  fn decode(tokens: impl AsRef<[Token]>) -> Result<Self, Self::Error>;
}

Then it's possible to implement the encoder and decoder for all rust builtin types like numbers (u64, u32), and structs (vectors, tuples, and arrays).

I wen with TryFrom for converting Token to rust types because that's what is used for FieldElement

Now, after I introduced Decode to convert a Vec<Token> to a struct, naming the trait that convert Token to a "scalar" type AbiDecode will be confusing, maybe using TryFrom is the best option after all, what do you guys think ?

[glihm]

@haroune-mohammedi @fracek it could be awesome to have your feedbacks on #475 related to this! As this one may be closed due to the overlap. :+1: