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Using the Substrate blockchain framework, how can I convert between Substrate specific types and Rust primitive types and vice versa?

For example:

  • Convert a time (T::Moment) to a u64
  • Convert a u64 to a T::Balance

etc...

Original: https://stackoverflow.com/questions/56081117/how-do-you-convert-between-substrate-specific-types-and-rust-primitive-types

1 Answer 1

7

For most Substrate specific types like BlockNumber, Balance, Nonce, etc... we guarentee that these types use a u32 with the AtLeast32Bit trait.

From the trait BaseArithmetic, the following are implemented:

  • From: u8, u16, u32
  • TryFrom: u64, u128, usize
  • TryInto: u8, u16, u32, u64, u128, usize

Another trait is also provided to provide ergonomic infallible conversion when you don't care if the value saturates.

  • UniqueSaturatedInto: u8, u16, u32, u64, u128
  • UniqueSaturatedFrom: u64, u128

NOTE on SaturatedConversion

SaturatedConversion (saturated_into and saturated_from) should not be used unless you know what you're doing, you've thought and considered all options and your use-case implies that saturation is fundamentally correct. The only time I imagine this is the case is deep in runtime arithmetic where you are logically certain it will not overflow, but can't provide a proof because it would depend on consistent pre-existing state.

This means that working from u32 to Substrate specific types should be easy:

pub fn u32_to_balance(input: u32) -> T::Balance {
    input.into()
}

For larger types, you need to handle the case where the Balance type for a runtime is smaller than what is available:

pub fn u64_to_balance_option(input: u64) -> Option<T::Balance> {
    input.try_into().ok()
}

// Note the warning above about saturated conversions
pub fn u64_to_balance_saturated(input: u64) -> T::Balance {
    input.saturated_into()
}

When converting from T::Balance to a rust primitive, you need to also handle conversion between incompatible types:

pub fn balance_to_u64(input: T::Balance) -> Option<u64> {
    TryInto::<u64>::try_into(input).ok()
}

// Note the warning above about saturated conversions
pub fn balance_to_u64_saturated(input: T::Balance) -> u64 {
    input.saturated_into::<u64>()
}

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