Cast Functions

Requires feature = "udf". See the UDF overview for shared semantics.

#[duckdb_cast] is applied to a one-argument fn(source) -> target (or -> Result<target, E>). The source and target logical types are inferred from the signature. Casts are unnamed, so there is no name option.

use better_duck_core::duckdb_cast;

#[duckdb_cast]
fn parse_to_int(s: &str) -> Result<i32, std::num::ParseIntError> { s.trim().parse() }

CAST('42' AS INTEGER)       -- 42
TRY_CAST('oops' AS INTEGER) -- NULL (a per-row conversion error becomes NULL)
CAST('oops' AS INTEGER)     -- query error (a normal CAST fails the query)

The same body serves both CAST and TRY_CAST. Under TRY_CAST, a conversion error (an Err return) sets that output row to NULL; under a normal CAST, it fails the query with the error’s message.

Options

Option	Meaning
`crate = ::path`	retarget the generated `::better_duck_core` path
`implicit_cost = <int>`	the binder’s implicit-cast cost: lower = preferred; negative = explicit-only. Defaults to `-1` (explicit `CAST` only).

A lower non-negative implicit_cost makes DuckDB more willing to apply the cast implicitly (without an explicit CAST). implicit_cost must be an integer literal (optionally negative).

The `VCast` trait

The macro implements better_duck_core::udf::VCast:

VCast by hand

// type Shared: Send + Sync + 'static;   — registration-time shared state (use ())
// fn source_type() -> Result<LogicalType>;
// fn target_type() -> Result<LogicalType>;
// fn implicit_cast_cost() -> i64 { -1 }
// fn cast_row(shared, input: &VectorRef, output: &mut VectorMut, row: usize) -> UdfResult<()>;

Register with conn.register_cast_function::<C>() or register_cast_function_with_state::<C>(shared). The same cast_row body serves CAST (a conversion error fails the query) and TRY_CAST (a conversion error yields NULL for that row). Work over VectorRef / VectorMut.

Connecting

Querying

Values & Types

Parameters & Statements

Writing Data

Columnar

Concurrency

Interop

Errors & Extensions

User-Defined Functions

Derives & Helpers

Options

The `VCast` trait

See also

​Options

​The VCast trait

​See also

Options

The `VCast` trait

See also