Return a new download.

Return a procedure that can be passed to 'build-event-output-port'. That procedure computes the new build status upon each event and calls ON-CHANGE:

(ON-CHANGE event status new-status)

ON-CHANGE can display the build status, build events, etc.

Print information about EVENT and STATUS to PORT. When COLORIZE? is true, produce colorful output. When PRINT-LOG? is true, display the build log in addition to build events. When PRINT-URLS? is true, display the URL of substitutes being downloaded.

Given EVENT, a tuple like (build-started "/gnu/store/...-foo.drv" ...), compute a new status based on STATUS.

Return an output port for use as 'current-build-output-port' that calls PROC with its current state value, initialized with SEED, on every build event. Build events passed to PROC are tuples corresponding to the "build traces" produced by the daemon:

(build-started "/gnu/store/...-foo.drv" ...) (substituter-started "/gnu/store/...-foo" ...)

and so on.

The second return value is a thunk to retrieve the current state.