code:

;This code example will walk you through some of the
;inane bullshit resulting from coersion and $T
;i'm gonna use the full all-caps versions of the various
;mumps commands since the more common single-letter
;versions always confuse people
;
thisTagIsNotBroken NEW foo
 SET foo=1
 IF 'foo DO clobberDoIT^THISROUTINE ;' == ! btw
 ELSE 
 .WRITE "i elsed"
 QUIT
;So this tag is the normal bool eval you expect.
;it will print "i elsed"
;
thisTagIsNotBrokenButItsWeird NEW foo
 SET foo="1string"
 IF 'foo DO clobberDoIt^THISROUTINE
 ELSE
 .WRITE "i elsed"
 QUIT
;This tage coerces "1string" to a number, truncing it to 1,
; then nots it, turning it to 0. this sets $T to 0 because
; that's essentially the same as "if false"
; so this tag also just prints "i elsed"
;
thisTagIsBroken NEW foo
 SET foo=1
 IF foo DO clobberDolT^THISROUTINE ; foo is 1, so $T is set to 1, so we do the tag
 ELSE ;clobberDolT set $T to 0 with its bogus IF, ELSE is satisfied so we enter the else block
 .WRITE "why am i here?" ;this prints
 QUIT
;This tag will always execute both sides of the if-else
;because the global truth variable gets clobbered inside the DO
;resulting in the ELSE reading that false state and executing
;
thisTagIsFixedByIDIOMS NEW foo
 SET foo=1
 IF foo DO clobberDolT^THISROUTINE IF 1 ;IF 1 doesn't execute if the initial if fails
 ELSE
 .WRITE "i can't get here"
 QUIT
;The "i 1" idiom is a defensive coding practice to save you from your d blocks
;clobbering $T. If a boolean evaluation is false, the rest of the line is skipped
;but if it's true, the DO is executed and then we return to the line afterward
;and execute the "i 1", setting $T to 1 and ensuring we don't fall thorugh
;to the ELSE. this bites people so much that Epic's custom in-house linter
;won't let you check in code without this idiom


clobberDolT
 IF 0=1
 QUIT