Why does this call invoke (read in-make-pipe) a block in a racket?

Question

I have this code from the documentation:

(define-values (in out) (make-pipe))
(write "234234" out)
(read in)

This creates "234234", as in the docs. This next piece of code simply blocks reading. Why is this happening?

(define-values (in out) (make-pipe))
(write 234234 out) ; <-- NOT A STRING
(read in) ;          <-- BLOCKS

Answer 1

The main problem is that the reader must parse the full value from the input. When you send "234234" to the pipe, the pipe contains 8 characters, and the last (second double quote) indicates readthat the value is complete. When you write 234234, the only thing in the pipe is the numbers, and the reader cannot determine if that number is complete. To see this, try the following:

#lang racket

(define-values (in out) (make-pipe))
(write 234234 out)
(write 111 out)
(display " " out)
(read in)

this calls the number 234234111.