Macro define 

macro_rules! define {
    () => { ... };
    ($try_transform:ident) => { ... };
    ($try_transform:ident, $try_inverse:ident) => { ... };
    ($param_var:ident: $ParamType:ty) => { ... };
    ($param_var:ident: $ParamType:ty, $try_transform:ident) => { ... };
    ($param_var:ident: $ParamType:ty, $try_transform:ident, $try_inverse:ident) => { ... };
}

Define the boilerplate for the Lens.

This macro is the easiest way to setup your Lens file - provide this with a transform function and you should have a valid Lens.

This macro handles multiple patterns, each handled pattern will generate a different set of functions.

• next() and alloc() will always be defined.
• transform() will be defined if given a single transform function.
• transform() and inverse() will be defined if given the identities of two transform/inverse functions.
• set_param() will be defined if given the name of a variable to hold a param, and its type.

Examples

The following example contains a complete Lens that transforms inputs by incrementing input.age by 1.

lens_sdk::define!(try_transform);

#[derive(Serialize, Deserialize)]
pub struct Value {
    pub name: String,
	pub age: i64,
}

fn try_transform(
    iter: &mut dyn Iterator<Item = lens_sdk::Result<Option<Value>>>,
) -> Result<StreamOption<Value>, Box<dyn Error>> {
    for item in iter {
        let input = match item? {
            Some(v) => v,
            None => return Ok(StreamOption::None),
        };

        let result = Value {
            name: input.name,
            age: input.age + 1,
        };

        return Ok(StreamOption::Some(result))
    }

    Ok(StreamOption::EndOfStream)
}

The next example contains a complete Lens that transforms, and inverses, the incrementing of input.age by the amount provided by the configured paramter.

lens_sdk::define!(PARAMETERS: Parameters, try_transform, try_inverse);

#[derive(Deserialize, Clone)]
pub struct Parameters {
    pub magnitude: i64,
}

static PARAMETERS: RwLock<Option<Parameters>> = RwLock::new(None);

#[derive(Serialize, Deserialize)]
pub struct Value {
    pub name: String,
	pub age: i64,
}

fn try_transform(
    iter: &mut dyn Iterator<Item = lens_sdk::Result<Option<Value>>>,
) -> Result<StreamOption<Value>, Box<dyn Error>> {
    let params = PARAMETERS.read()?
        .clone()
        .ok_or(LensError::ParametersNotSetError)?;

    for item in iter {
        let input = match item? {
            Some(v) => v,
            None => return Ok(StreamOption::None),
        };

        let result = Value {
            name: input.name,
            age: input.age + params.magnitude,
        };

        return Ok(StreamOption::Some(result))
    }

    Ok(StreamOption::EndOfStream)
}

fn try_inverse(
    iter: &mut dyn Iterator<Item = lens_sdk::Result<Option<Value>>>,
) -> Result<StreamOption<Value>, Box<dyn Error>> {
    let params = PARAMETERS.read()?
        .clone()
        .ok_or(LensError::ParametersNotSetError)?;

    for item in iter {
        let input = match item? {
            Some(v) => v,
            None => return Ok(StreamOption::None),
        };

        let result = Value {
            name: input.name,
            age: input.age - params.magnitude,
        };

        return Ok(StreamOption::Some(result))
    }

    Ok(StreamOption::EndOfStream)
}