io

The io module exports the stream shapes as the traits Reader and Writer. Every standard library API that consumes a stream, such as an HTTP request body, exec.Command(stdout=...), or the dest= keyword of csv.write_all and the compression and archive modules, accepts a value that implements the trait as well as one that simply defines the methods.

Trait	Required	Optional	Contract
`Reader`	`read(self, size)`	`close(self)`	Return at most `size` bytes as Bytes or str. An empty chunk or `nil` ends the stream.
`Writer`	`write(self, data)`	`close(self)`	Receive a Bytes chunk and return the number of bytes written, or `nil` for all of them.

close does nothing by default. Consumers that own a stream call it when they finish; writer consumers never do, because the stream's owner decides when it ends.

import "io"
import "csv"

type Collector(chunks) {
    impl io.Writer {
        func write(self, data) {
            self.chunks.push(data.decode())
            return data.size
        }
    }
}

var out = Collector([])
csv.write_all([["name", "score"], ["Ada", "10"]], dest=out)
print(out.chunks.join(""))

As with every trait, write is not an attribute of the instance: out.write(x) does not exist, and the method is called as io.Writer.write(out, x). Use plain read/write methods instead when callers should be able to call them directly. The trait calls work on any stream, not just on impls: io.Reader.read(file, 10) reads from an fs file, and it also runs the plain read method of a user type that has one.

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