Michał “phoe” Herda

Michał “phoe” Herda

[email protected]://phoe.github.io

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• We need means of unwinding and protecting against unwinds

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• We need means of unwinding and protecting against unwinds

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• We need means of unwinding and protecting against unwinds• We can implement all of CL-specific control flow with this!

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• if• tagbody/go• block/return-from• catch/throw• unwind-protect• lambda/apply

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• This includes non-local jumps(a secret Common Lisp feature)



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• This includes loops• This includes switches• This includes error handling• This includes restart handling



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• This includes loops• This includes switches• This includes error handling• This includes restart handling• The above four groups

are implemented in Common Lisp itself

• Software engineer at Ericsson (C/C++/Erlang)

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• Common Lisp enthusiast (#lisp, Lisp Discord, Reddit)

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• Common Lisp enthusiast (#lisp, Lisp Discord, Reddit)• Author of The Common Lisp Condition System (Apress, 2020)

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• Somewhat capable of writing other programming languages

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• Somewhat capable of writing other programming languages

• I kinda like the topic of control flow

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foo()

bar()

baz()

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foo()

bar()

baz()

1/0

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foo()

bar()

baz()

1/0

ArithmeticException

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foo()

bar()

baz()

1/0

ArithmeticException

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foo()

bar()

baz()

1/0

ArithmeticException

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foo()

bar()

baz()

1/0

ArithmeticException

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foo()

bar()

baz()

1/0

catch

ArithmeticException

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foo()

bar()

baz()

1/0

catch

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foo()

bar()

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foo()

bar()

... // execution continues

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(foo)

(bar)

(baz)

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(foo)

(bar)

(baz)

(/ 1 0)

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(foo)

(bar)

(baz)

(/ 1 0)division-by-zero

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(foo)

(bar)

(baz)


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(foo)

(bar)

(baz)


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(foo)

(bar)

(baz)


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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero

(signal ...)

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero

(signal ...)

handler-3

handler-2

handler-1

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero

(signal ...)

handler-3

handler-2

handler-1

handler-1

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero

(signal ...)

handler-3

handler-2

handler-1

handler-2

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero

(signal ...)

handler-3

handler-2

handler-1

handler-3

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero

(signal ...)

handler-3

handler-2

handler-1

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero

(invoke-debugger ...)

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero


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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero


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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

handler-2

handler-3

handler-2

handler-1

division-by-zero

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

handler-2

handler-3

handler-2

handler-1

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

handler-2

handler-3

handler-2

handler-1

non-local jump

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(foo)

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(foo)


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(foo)


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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero


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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero


restart-1restart-2

restart-4

restart-3

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero


“Return 42 instead.”“Query the user for new numbers.”

“Abort and return to toplevel.”

“Try opening another file.”

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero





?

?

??

?

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)





division-by-zero

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

division-by-zero

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

handler-2

handler-3

handler-2

handler-1

division-by-zero

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

handler-2

handler-3

handler-2

handler-1

division-by-zero


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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

handler-2

handler-3

handler-2

handler-1

division-by-zero


restart-fn

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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

handler-2

handler-3

handler-2

handler-1

division-by-zero


restart-fn

non-localjump

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(foo)

(bar)

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(foo)

(bar)


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(foo)

(bar)

(baz)

(quux)

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(foo)

(bar)

(baz)

(quux)some-condition

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(foo)

(bar)

(baz)


(signal ...)

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(foo)

(bar)

(baz)


handler-3

handler-2

handler-1

(signal ...)

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(foo)

(bar)

(baz)


handler-3

handler-2

handler-1

(signal ...)

handler-1

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(foo)

(bar)

(baz)


handler-3

handler-2

handler-1

(signal ...)

handler-2

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(foo)

(bar)

(baz)


handler-3

handler-2

handler-1

(signal ...)

handler-3

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(foo)

(bar)

(baz)


handler-3

handler-2

handler-1

(signal ...)

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(foo)

(bar)

(baz)


handler-3

handler-2

handler-1

¯\_(ツ)_/¯

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(foo)

(bar)

(baz)

(quux)

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(foo)

(bar)

(baz)

(quux)


• Construct the exception object

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• Construct the exception object• Unwind the stack immediately

• Stop unwinding when something catches the exception

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• Construct the exception object• Unwind the stack immediately

• Stop unwinding when something catches the exception• Continue execution from that point

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• Construct the condition object

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• Construct the condition object• Call handlers from the stack in order

• What do the handlers do?

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• What do the handlers do?

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• What do the handlers do?• Maybe execute some code

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• What do the handlers do?• Maybe execute some code• Maybe invoke a restart

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• What do the handlers do?• Maybe execute some code• Maybe invoke a restart• Maybe do nothing and return

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• What do the handlers do?• Maybe execute some code• Maybe invoke a restart• Maybe do nothing and return• Maybe unwind the stack to a predefined point

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• What do the handlers do?• Maybe execute some code• Maybe invoke a restart• Maybe do nothing and return• Maybe unwind the stack to a predefined point• Maybe there are no handlers

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• What do the handlers do?• Maybe execute some code• Maybe invoke a restart• Maybe do nothing and return• Maybe unwind the stack to a predefined point• Maybe there are no handlers• Maybe she’s born with it

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• What do the handlers do?• Maybe execute some code• Maybe invoke a restart• Maybe do nothing and return• Maybe unwind the stack to a predefined point• Maybe there are no handlers• Maybe she’s born with it• Maybe it’s Maybelline™

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• If there was no transfer of control, return• ...and maybe enter the debugger to halt the program

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• signaling = a dynamically scoped hooking mechanism

• progress bars• message passing• calling asynchronous code• etc..




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• restarts = a dynamically scoped mechanism of choices

• context-dependent actionsfor interactive programming

• context-dependent meansof automated error recovery(e.g. when parsing incomplete source code)

• etc..




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• etc..




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• etc..




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• etc..




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• etc..




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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

handler-2




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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

handler-2




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(foo)

(bar)

(baz)

(/ 1 0)

(error ...)

handler-2

• if

(if (foo) (bar) (baz))

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• if• tagbody/go

(tagbody 10 (print “hello”) 20 (go 10))

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(block my-block (...) (... (return-from my-block 42)) (...))

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• if• tagbody/go• block/return-from

(catch ‘quux (...) (... (foo)) (...)))

(defun foo () (throw ‘quux 42))

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• if• tagbody/go• block/return-from• catch/throw

• if• tagbody/go• block/return-from• catch/throw• unwind-protect

(let ((thing (make-thing))) (unwind-protect (frob thing) (cleanup thing)))

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(let ((fn (lambda ...)) (args ...)) (apply fn 1 2 3 args))

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(let ((fn (lambda ...)) (args ...)) (apply fn 1 2 3 args))

(let ((fn (lambda ...))) (funcall fn 1 2 3))

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• if• tagbody/go• block/return-from• catch/throw• unwind-protect• lambda/apply ; and funcall

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(let ((x 42)) (lambda () x))

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(let ((x 42)) (lambda () x));; #<FUNCTION (LAMBDA ())>

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(funcall *) ; (funcall #<FUNCTION (LAMBDA ())>)

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(funcall *) ; (funcall #<FUNCTION (LAMBDA ())>);; => 42

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;; but we can close over more;; than just lexical variables!

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;; but we can close over more;; than just lexical variables!

(defun foo (x) (funcall x))

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(defun bar () ; block bar ...)

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(defun bar () (let ((fn (lambda () (return-from bar 42)))) ...))

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(defun bar () (let ((fn (lambda () (return-from bar 42)))) (foo fn)))

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(bar)

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(bar)(bar)

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(bar)(bar)

(foo)

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(bar)(bar)

(foo)

(lambda () ...)

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(bar)(bar)

(foo)

(lambda () ...)

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(bar)(bar)

(foo)

(lambda () ...)

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(bar);; => 42

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(bar);; => 42

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(foo)

(bar)

(baz)

(quux)

(frob)

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(foo)

(bar)

(baz)

(quux)

(frob)

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(foo)

(bar)

(baz)

(quux)

(frob)

1

!

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(foo)

(bar)

(baz)

(quux)

(frob)

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(foo)

(bar)

(baz)

(quux)

(frob)1

?

?

?

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(foo)

(bar)

(baz)

(quux)

(frob)1

?

?

?

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(foo)

(bar)

(baz)

(quux)

(frob)1

2?

?

?

!

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• if• tagbody/go ; 1-phase unwind (no search)• block/return-from ; 1-phase unwind (no search)• catch/throw ; 2-phase unwind (search)• unwind-protect• lambda/apply

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• All other CL control flow operators are derivatives of those primitives

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• All other CL control flow operators are derivatives of those primitives• loops (do, dolist, loop, ...)

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• All other CL control flow operators are derivatives of those primitives• loops (do, dolist, loop, ...)• switches (cond, case, typecase, ...)

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• All other CL control flow operators are derivatives of those primitives• loops (do, dolist, loop, ...)• switches (cond, case, typecase, ...)• error handling (handler-case, ...)

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• All other CL control flow operators are derivatives of those primitives• loops (do, dolist, loop, ...)• switches (cond, case, typecase, ...)• error handling (handler-case, ...)• restarts (restart-case, ...)

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• This list includes use cases that are not related to exception handling

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• Control flow ≠ exception handling




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• Do not conflate unwinding with throwing exceptions

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• Do not conflate unwinding with throwing exceptions• Throwing exceptions is a subset of control flow

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• Do not conflate unwinding with throwing exceptions• Throwing exceptions is a subset of control flow• Throwing exceptions is not synonymous with unwinding

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• Do not conflate unwinding with throwing exceptions• Throwing exceptions is a subset of control flow• Throwing exceptions is not synonymous with unwinding• Throwing exceptions is not a primitive operation

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• Do not conflate unwinding with throwing exceptions• Throwing exceptions is a subset of control flow• Throwing exceptions is not synonymous with unwinding• Throwing exceptions is not a primitive operation

• Proofs: Common Lisp, Dylan, Smalltalk

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but wait hold on for just one moment

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Appendix A

Proving one-phase unwind in TAGBODY and BLOCK

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(block foo (lambda () (return-from foo)))

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(let ((fn (block foo (lambda () (return-from foo))))) (funcall fn)); ...?

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(let ((fn (block foo (lambda () (return-from foo))))) (funcall fn)); ERROR: Condition CONTROL-ERROR; was signaled.

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• if• tagbody/go ; 1-phase unwind (no search)• block/return-from ; 1-phase unwind (no search)• catch/throw ; 2-phase unwind (search)• unwind-protect• lambda/apply (let ((fn (block foo

(lambda () (return-from foo))))) (funcall fn)); ERROR: Condition CONTROL-ERROR; was signaled.

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• if• tagbody/go ; 1-phase unwind (no search)• block/return-from ; 1-phase unwind (no search)• catch/throw ; 2-phase unwind (search)• unwind-protect• lambda/apply (let ((fn (block foo

(lambda () (return-from foo))))) (funcall fn)); ERROR: Condition CONTROL-ERROR; was signaled.

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? ?

(block foo (lambda () (return-from foo)))

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(block foo ...)

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(let ((return-valid-p t)) (unwind-protect (%unwind-tag foo ...) (setf return-valid-p nil)))

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(let ((return-valid-p t)) (unwind-protect (%unwind-tag foo ...) ;; let’s expand the lambda! (setf return-valid-p nil)))

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(let ((return-valid-p t)) (unwind-protect (%unwind-tag foo (lambda () (if return-valid-p (%1-phase-unwind-to-tag foo) (error ‘control-error)))) (setf return-valid-p nil)))

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;; similar validation scheme applies for TAGBODY/GO

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Appendix B

Describing UNWIND-PROTECT

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(foo)

(bar)

(baz)

(quux)

(frob)

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(foo)

(bar)

(baz)

(quux)

(frob)

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(foo)

(bar)

(baz)

(quux)

(frob)

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(foo)

(bar)

(baz)

(quux)

(frob)

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(foo)

(bar)

(baz)

(quux)

(frob)

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(foo)

(bar)

(baz)

(quux)

(frob)

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(foo)

(bar)

(baz)

(quux)

(frob)

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Appendix C

Common Lisp condition system without Common Lisp

(this is the last one I promise)199

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• if• tagbody/go• block/return-from• catch/throw• try/finally• new/.apply()• throw exception

205• Can we port the condition system to Java?

206• Can we port CL control flow to Java?

207• Can we port CL control flow to Java?

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“I suppose it is tempting, if the only tool you have is a hammer,to treat everything as if it were a nail.”

--- Abraham H. Maslow

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https://github.com/phoe/cafe-latte

yes, it’s seriously the end this time

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Michał “phoe” Herda

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