Such terrible news, in fact, that I can't even think what to say without sounding maudlin. Williams had been struggling with depression and cocaine and alcohol addiction for many years.
I read an excellent article from the Hollywood Reporter (link below) quoting Williams as he talked openly about his addiction problems. He quit for the first time in 1982 after being with John Belushi the night of that star's fatal overdose on cocaine and heroin.
Williams confessed to relapsing and sought treatment for alcohol addiction several times, including just recently at a Minnesota rehab clinic. In interviews, he joked about how quickly his drinking problem escalated, saying one pint of Jack Daniels soon led to so many bottles he sounded like a 'wind chime' walking down the street.
Here's more from that The Hollywood Reporter article. And an ABC News report about his long struggle with addiction, including an interview with Diane Sawyer.
Rest in peace - and joy. Because that's what you brought us. :( xo