Struct hcl::eval::FuncDef

pub struct FuncDef { /* private fields */ }

The definition of a function that can be called in HCL expressions.

It defines the function to call, and number and types of parameters that the function accepts. The parameter information is used to validate function arguments prior to calling it.

The signature of a function is defined by the Func type alias. For available parameter types see the documentation of ParamType.

Function call evaluation

When a [FuncCall][crate::structure::FuncCall] is evaluated (via its evaluate method), the arguments are validated against the defined function parameters before calling the function. The evaluation will stop with an error if too few or too many arguments are provided, of if their types do not match the expected parameter types.

Because all arguments are validated before calling the function, unnecessary length and type checks on the function arguments can be avoided in the function body.

Examples

use hcl::eval::{Context, FuncArgs, FuncDef, ParamType};
use hcl::Value;

fn add(args: FuncArgs) -> Result<Value, String> {
    let a = args[0].as_number().unwrap();
    let b = args[1].as_number().unwrap();
    Ok(Value::Number(*a + *b))
}

let params = [ParamType::Number, ParamType::Number];

let func_def = FuncDef::new(add, params);

let mut ctx = Context::new();

// Declare the function in the context to make it available
// during expression evaluation.
ctx.declare_func("add", func_def);

// Use the context to evaluate an expression.
// ...

Alternatively, the FuncDefBuilder can be used to construct the FuncDef:

let func_def = FuncDef::builder()
    .param(ParamType::Number)
    .param(ParamType::Number)
    .build(add);

See the documentation of the FuncDefBuilder for all available methods.

Implementations

impl FuncDef

pub fn new<P>(func: Func, params: P) -> FuncDefwhere
    P: IntoIterator<Item = ParamType>,

Creates a new FuncDef from a function and its parameters.

Note: if you want to define a FuncDef with a variadic parameter, use the .builder() method. It provides a FuncDefBuilder which also lets you define variadic parameters.

See the type-level documentation of FuncDef for usage examples.

pub fn builder() -> FuncDefBuilder

Creates a FuncDefBuilder.

See the type-level documentation of FuncDef for usage examples.

Trait Implementations

impl Clone for FuncDef

fn clone(&self) -> FuncDef

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for FuncDef

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.