CSV and JSON

Coral.Io reads and writes frames with CSV or JSON files. Its readers infer integer (i64), float (f32), bool, or string columns from the values in each column. A CSV file uses a header row. A JSON file must contain an array of objects with scalar values. File operations use Chelis's IO effect.

Format	Read	Write
CSV	read_csv_frame(path)	write_csv_frame(frame, path)
JSON	read_json_frame(path)	write_json_frame(frame, path)

The following test uses simple column names, plain text, finite floats, and no missing integers. In your Reef project, replace Coral in the module declaration with your project's module prefix. Save the program as src/bookio.ch, then run chelis test src/bookio.ch. It writes book-io.csv and book-io.json in the working directory and checks that each file reads back as a four-column frame:

module Coral.BookIo
import Std.Test (assert_eq)
import Coral.Frame (FloatCol, StringCol, BoolCol, from_pairs, ncols, int_col_of_list)
import Coral.Io (write_csv_frame, read_csv_frame, write_json_frame, read_json_frame)
def test_files() -> unit ! { Test, IO } = {
  frame = from_pairs([("id", int_col_of_list([1i64, 2i64])), ("price", FloatCol(to_tensor([10.0f32, 20.5f32]))), ("flag", BoolCol(to_tensor([true, false]))), ("city", StringCol(["london", "paris"]))])
  _ = write_csv_frame(frame, "book-io.csv")
  _ = write_json_frame(frame, "book-io.json")
  csv_frame = read_csv_frame("book-io.csv")
  json_frame = read_json_frame("book-io.json")
  _ = assert_eq(ncols(csv_frame), 4i64, "CSV has four columns")
  assert_eq(ncols(json_frame), 4i64, "JSON has four columns")
}

The CSV file contains:

id,price,flag,city
1,10.0,true,london
2,20.5,false,paris

The JSON file contains:

[{"id":1,"price":10.0,"flag":true,"city":"london"},{"id":2,"price":20.5,"flag":false,"city":"paris"}]

Before writing data, check these constraints:

• The CSV writer quotes commas and double quotes in cell values, but does not quote headers. Use simple column names. Its line-based reader cannot round-trip embedded CR or LF in a field.
• The JSON writer inserts column names and string values without JSON escaping. Avoid quotes, backslashes, and control characters in those strings. Use finite float values; a non-finite float is not valid JSON.
• Both writers output the underlying number for an IntCol and ignore its missing-value mask. A missing integer stored as 0 is written as 0, and reading the file back does not restore the mask.
• CSV and JSON readers infer each column from its cell text. A StringCol containing only digit strings reads back as an IntCol, which can discard leading zeroes. An integer outside i64 may read back as FloatCol with rounding. Do not rely on these readers to preserve digit-only identifiers.

read_parquet_frame and write_parquet_frame are exported names but fail when called. Parquet frame I/O is unavailable.