tobixdev commented on code in PR #8829:
URL: https://github.com/apache/arrow-rs/pull/8829#discussion_r2520069266
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arrow-cast/src/pretty.rs:
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@@ -22,14 +22,99 @@
//! [`RecordBatch`]: arrow_array::RecordBatch
//! [`Array`]: arrow_array::Array
-use std::fmt::Display;
-
use comfy_table::{Cell, Table};
+use std::fmt::{Display, Write};
+use std::sync::Arc;
-use arrow_array::{Array, ArrayRef, RecordBatch};
-use arrow_schema::{ArrowError, SchemaRef};
+use arrow_array::cast::AsArray;
+use arrow_array::{Array, ArrayRef, Int32Array, RecordBatch, array};
+use arrow_schema::{ArrowError, Field, SchemaRef};
-use crate::display::{ArrayFormatter, FormatOptions};
+use crate::display::{ArrayFormatter, DisplayIndex, FormatOptions};
+
+/// Allows creating a new [`ArrayFormatter`] for a given [`Array`] and an
optional [`Field`].
+///
+/// # Example
+///
+/// The example below shows how to create a custom formatter for a custom type
`my_money`.
+///
+/// ```rust
+/// use std::fmt::Write;
+/// use arrow_array::{Array, Int32Array, cast::AsArray};
+/// use arrow_cast::display::{ArrayFormatter, DisplayIndex, FormatOptions,
FormatResult};
+/// use
arrow_cast::pretty::{pretty_format_batches_with_options_and_formatters,
ArrayFormatterFactory};
+/// use arrow_schema::{ArrowError, Field};
+///
+/// /// A custom formatter factory that can create a formatter for the special
type `my_money`.
+/// ///
+/// /// This struct could have access to some kind of extension type registry
that can lookup the
+/// /// correct formatter for an extension type on-demand.
+/// struct MyFormatters {}
+///
+/// impl ArrayFormatterFactory for MyFormatters {
+/// fn create_display_index<'formatter>(
+/// &self,
+/// array: &'formatter dyn Array,
+/// options: &'formatter FormatOptions<'formatter>,
+/// field: Option<&'formatter Field>,
+/// ) -> Result<Option<ArrayFormatter<'formatter>>, ArrowError> {
+/// // check if this is the money type
+/// if field
+/// .map(|f| f.extension_type_name() == Some("my_money"))
+/// .unwrap_or(false)
+/// {
+/// // We assume that my_money always is an Int32.
+/// let array = array.as_primitive();
+/// let display_index = Box::new(MyMoneyFormatter { array, options
});
+/// return Ok(Some(ArrayFormatter::new(display_index,
options.safe)));
+/// }
+///
+/// Ok(None) // None indicates that the default formatter should be
used.
+/// }
+/// }
+///
+/// /// A formatter for the type `my_money` that wraps a specific array and
has access to the
+/// /// formatting options.
+/// struct MyMoneyFormatter<'a> {
Review Comment:
From the DataFusion perspective, this would be a struct that users implement
such that they can pretty-print their custom types.
We would also do something similar for canonical data types (e.g.,
formatting UUIDs).
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