scale-decode

This crate makes it easy to decode SCALE encoded bytes into a custom data structure with the help of scale_info types. By using this type information to guide decoding (instead of just trying to decode bytes based on the shape of the target type), it's possible to be much more flexible in how data is decoded and mapped to some target type.

The main trait used to decode types is a Visitor trait (example below). By implementing this trait, you can describe how to take SCALE decoded values and map them to some custom type of your choosing (whether it is a dynamically shaped type or some static type you'd like to decode into). Implementations of this Visitor trait exist for many existing Rust types in the standard library.

There also exists an IntoVisitor trait, which is implemented on many existing rust types and maps a given type to some visitor implementation capable of decoding into it.

Finally, a wrapper trait, DecodeAsType, is auto-implemented for all types that have an IntoVisitor implementation, and whose visitor errors can be turned into a standard crate::Error.

For custom structs and enums, one can use the DecodeAsType derive macro to have a DecodeAsType implementation automatically generated.

Here's an example of implementing Visitor to decode bytes into a custom Value type:

use scale_decode::visitor::{self, TypeId};

// A custom type we'd like to decode into:
#[derive(Debug, PartialEq)]
enum Value {
    Bool(bool),
    Char(char),
    U8(u8),
    U16(u16),
    U32(u32),
    U64(u64),
    U128(u128),
    U256([u8; 32]),
    I8(i8),
    I16(i16),
    I32(i32),
    I64(i64),
    I128(i128),
    I256([u8; 32]),
    CompactU8(u8),
    CompactU16(u16),
    CompactU32(u32),
    CompactU64(u64),
    CompactU128(u128),
    Sequence(Vec<Value>),
    Composite(Vec<(String, Value)>),
    Tuple(Vec<Value>),
    Str(String),
    Array(Vec<Value>),
    Variant(String, Vec<(String, Value)>),
    BitSequence(scale_bits::Bits),
}

// Implement the `Visitor` trait to define how to go from SCALE
// values into this type:
struct ValueVisitor;
impl visitor::Visitor for ValueVisitor {
    type Value<'scale> = Value;
    type Error = visitor::DecodeError;

    fn visit_bool<'scale>(
        self,
        value: bool,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::Bool(value))
    }
    fn visit_char<'scale>(
        self,
        value: char,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::Char(value))
    }
    fn visit_u8<'scale>(
        self,
        value: u8,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::U8(value))
    }
    fn visit_u16<'scale>(
        self,
        value: u16,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::U16(value))
    }
    fn visit_u32<'scale>(
        self,
        value: u32,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::U32(value))
    }
    fn visit_u64<'scale>(
        self,
        value: u64,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::U64(value))
    }
    fn visit_u128<'scale>(
        self,
        value: u128,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::U128(value))
    }
    fn visit_u256(
        self,
        value: &'_ [u8; 32],
        _type_id: TypeId,
    ) -> Result<Self::Value<'_>, Self::Error> {
        Ok(Value::U256(*value))
    }
    fn visit_i8<'scale>(
        self,
        value: i8,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::I8(value))
    }
    fn visit_i16<'scale>(
        self,
        value: i16,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::I16(value))
    }
    fn visit_i32<'scale>(
        self,
        value: i32,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::I32(value))
    }
    fn visit_i64<'scale>(
        self,
        value: i64,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::I64(value))
    }
    fn visit_i128<'scale>(
        self,
        value: i128,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::I128(value))
    }
    fn visit_i256(
        self,
        value: &'_ [u8; 32],
        _type_id: TypeId,
    ) -> Result<Self::Value<'_>, Self::Error> {
        Ok(Value::I256(*value))
    }
    fn visit_compact_u8<'scale>(
        self,
        value: Compact<u8>,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::CompactU8(value.value()))
    }
    fn visit_compact_u16<'scale>(
        self,
        value: Compact<u16>,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::CompactU16(value.value()))
    }
    fn visit_compact_u32<'scale>(
        self,
        value: Compact<u32>,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::CompactU32(value.value()))
    }
    fn visit_compact_u64<'scale>(
        self,
        value: Compact<u64>,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::CompactU64(value.value()))
    }
    fn visit_compact_u128<'scale>(
        self,
        value: Compact<u128>,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::CompactU128(value.value()))
    }
    fn visit_sequence<'scale>(
        self,
        value: &mut Sequence<'scale, '_>,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        let mut vals = vec![];
        while let Some(val) = value.decode_item(ValueVisitor) {
            let val = val?;
            vals.push(val);
        }
        Ok(Value::Sequence(vals))
    }
    fn visit_composite<'scale>(
        self,
        value: &mut Composite<'scale, '_>,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        let mut vals = vec![];
        for item in value.by_ref() {
            let item = item?;
            let val = item.decode_with_visitor(ValueVisitor)?;
            let name = item.name().unwrap_or("").to_owned();
            vals.push((name, val));
        }
        Ok(Value::Composite(vals))
    }
    fn visit_tuple<'scale>(
        self,
        value: &mut Tuple<'scale, '_>,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        let mut vals = vec![];
        while let Some(val) = value.decode_item(ValueVisitor) {
            let val = val?;
            vals.push(val);
        }
        Ok(Value::Tuple(vals))
    }
    fn visit_str<'scale>(
        self,
        value: &mut Str<'scale>,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        Ok(Value::Str(value.as_str()?.to_owned()))
    }
    fn visit_variant<'scale>(
        self,
        value: &mut Variant<'scale, '_>,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        let mut vals = vec![];
        let fields = value.fields();
        for item in fields.by_ref() {
            let item = item?;
            let val = item.decode_with_visitor(ValueVisitor)?;
            let name = item.name().unwrap_or("").to_owned();
            vals.push((name, val));
        }
        Ok(Value::Variant(value.name().to_owned(), vals))
    }
    fn visit_array<'scale>(
        self,
        value: &mut Array<'scale, '_>,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        let mut vals = vec![];
        while let Some(val) = value.decode_item(ValueVisitor) {
            let val = val?;
            vals.push(val);
        }
        Ok(Value::Array(vals))
    }
    fn visit_bitsequence<'scale>(
        self,
        value: &mut BitSequence<'scale>,
        _type_id: TypeId,
    ) -> Result<Self::Value<'scale>, Self::Error> {
        let bools: Result<scale_bits::Bits, _> = value.decode()?.collect();
        Ok(Value::BitSequence(bools?))
    }
}

This can then be passed to a decode function like so:

let value: Value = scale_decode::visitor::decode_with_visitor(scale_bytes, type_id, types, ValueVisitor)?;

Where scale_bytes are the bytes you'd like to decode, type_id is the type stored in the types registry that you'd like to try and decode the bytes into, and types is a scale_info::PortableRegistry containing information about the various types in use.

If we were to then write an IntoVisitor implementation like so:

impl scale_decode::IntoVisitor for Value {
    type Visitor = ValueVisitor;
    fn into_visitor() -> Self::Visitor {
        ValueVisitor
    }
}

We can then also decode via tha automatic DecodeAsType impl like so:

use scale_decode::DecodeAsType;

let value = Value::decode_as_type(scale_bytes, type_id, types)?;

With an IntoVisitor impl, you'd also benefit from being able to decode things like Vec<Value>, (Value, bool), Arc<Value> and so on in the same way.