author | Tony Garnock-Jones <tonygarnockjones@gmail.com> |
Wed, 16 Jan 2019 17:15:58 +0000 | |
changeset 438 | 1fe179d53161 |
parent 1 | e3e8313b3acc |
permissions | -rw-r--r-- |
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Interesting documents from Oleg about monads and exceptions
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From posting-system@google.com Mon Jun 10 17:16:34 2002 |
e3e8313b3acc
Interesting documents from Oleg about monads and exceptions
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Date: Mon, 10 Jun 2002 13:16:23 -0700 |
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Interesting documents from Oleg about monads and exceptions
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From: oleg@pobox.com (oleg@pobox.com) |
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Interesting documents from Oleg about monads and exceptions
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Newsgroups: comp.lang.scheme |
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Interesting documents from Oleg about monads and exceptions
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Subject: Re: multilambda implementation: recreational macrology challenge! |
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Interesting documents from Oleg about monads and exceptions
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References: <3CFE82D7.1355074F@sonic.net> <87lm9tgqh6.fsf@radish.petrofsky.org> <3CFEEA0F.6040009@sonic.net> |
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Interesting documents from Oleg about monads and exceptions
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Message-ID: <7eb8ac3e.0206101216.1eb26713@posting.google.com> |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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Status: OR |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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Interesting documents from Oleg about monads and exceptions
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This article demonstrates that a lexical-scoped exception handling is |
e3e8313b3acc
Interesting documents from Oleg about monads and exceptions
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possible to some extent. We shall also show how to add 1 to the |
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Interesting documents from Oleg about monads and exceptions
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_value_ of (car '()) and store the result, without terminating the |
e3e8313b3acc
Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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computation. |
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Interesting documents from Oleg about monads and exceptions
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Interesting documents from Oleg about monads and exceptions
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The technique of this article may be considered R5RS. The modality in |
e3e8313b3acc
Interesting documents from Oleg about monads and exceptions
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the previous sentence comes entirely from R5RS. Section 6.4 of R5RS |
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Interesting documents from Oleg about monads and exceptions
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specifically says "Calling force on an object that is not a promise |
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Interesting documents from Oleg about monads and exceptions
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may simply return the object; Some implementations may implement |
e3e8313b3acc
Interesting documents from Oleg about monads and exceptions
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'implicit forcing, where the value of a promise is forced by |
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Interesting documents from Oleg about monads and exceptions
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primitive procedures like cdr and +:". Such implementations do |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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exist. For example, in Gambit-3.0 a force applied to a value that is |
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Interesting documents from Oleg about monads and exceptions
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not a promise simply returns that value. Gambit-3.0 _compiler_ with an |
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Interesting documents from Oleg about monads and exceptions
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undocumented flag can implicitly force promises. In this article, we |
e3e8313b3acc
Interesting documents from Oleg about monads and exceptions
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will use Bigloo 2.4b system, which permits neither of these |
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Interesting documents from Oleg about monads and exceptions
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extensions. Therefore, we have to appropriately generalize force |
e3e8313b3acc
Interesting documents from Oleg about monads and exceptions
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Interesting documents from Oleg about monads and exceptions
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(set! force |
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Interesting documents from Oleg about monads and exceptions
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(let ((force force)) |
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Interesting documents from Oleg about monads and exceptions
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(lambda (x) (if (procedure? x) (force x) x)))) |
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Interesting documents from Oleg about monads and exceptions
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Interesting documents from Oleg about monads and exceptions
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and explicitly forcify +, -, car, cdr (but not cons or vector) |
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Interesting documents from Oleg about monads and exceptions
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procedures. We also assume SRFI-12 (which has been implemented for |
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Interesting documents from Oleg about monads and exceptions
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Bigloo and Gambit: http://pobox.com/~oleg/ftp/Scheme/util.html#srfi-12 |
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Interesting documents from Oleg about monads and exceptions
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Chicken also supports SRFI-12). |
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Interesting documents from Oleg about monads and exceptions
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Interesting documents from Oleg about monads and exceptions
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(define (force-catch proc . args) |
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Interesting documents from Oleg about monads and exceptions
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(handle-exceptions exc |
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Interesting documents from Oleg about monads and exceptions
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(begin (cerr "Exception: " exc nl) (delay (abort exc))) |
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Interesting documents from Oleg about monads and exceptions
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(apply proc (map force args)))) |
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Interesting documents from Oleg about monads and exceptions
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Interesting documents from Oleg about monads and exceptions
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(define (if-error x thunk) |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(handle-exceptions exc (thunk) (force x))) |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(set! + (let ((+ +)) (lambda args (apply force-catch (cons + args))))) |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(set! - (let ((- -)) (lambda args (apply force-catch (cons - args))))) |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(set! * (let ((* *)) (lambda args (apply force-catch (cons * args))))) |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(set! / (let ((/ /)) (lambda args (apply force-catch (cons / args))))) |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(set! car (let ((car car)) |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(lambda args (apply force-catch (cons car args))))) |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(set! cdr (let ((cdr cdr)) (lambda args (apply force-catch |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(cons cdr args))))) |
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Interesting documents from Oleg about monads and exceptions
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Interesting documents from Oleg about monads and exceptions
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With the preliminaries out of the way, let the fun begin. |
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Interesting documents from Oleg about monads and exceptions
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Interesting documents from Oleg about monads and exceptions
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(display "Fun Begins") (newline) |
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Interesting documents from Oleg about monads and exceptions
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Interesting documents from Oleg about monads and exceptions
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(display (+ 4 (/ 1 1))) (newline) |
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Interesting documents from Oleg about monads and exceptions
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(display |
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Interesting documents from Oleg about monads and exceptions
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(if-error (+ 4 (/ 1 0)) |
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Interesting documents from Oleg about monads and exceptions
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(lambda () "error"))) |
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Interesting documents from Oleg about monads and exceptions
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Interesting documents from Oleg about monads and exceptions
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We see "5" and "error" printed. One can say, what is the big deal? We |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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generate an exception and catch it at the appropriate moment. Well, |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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let's enter the following definition: |
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Interesting documents from Oleg about monads and exceptions
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Interesting documents from Oleg about monads and exceptions
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(define wv (vector 1 (/ 1 0) (car '()))) |
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Interesting documents from Oleg about monads and exceptions
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(display "OK\n") |
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Interesting documents from Oleg about monads and exceptions
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Interesting documents from Oleg about monads and exceptions
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The computation completed without any error! We can go further, and |
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Interesting documents from Oleg about monads and exceptions
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add 1 to each element of the above vector: |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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Interesting documents from Oleg about monads and exceptions
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(define wl (map (lambda (x) (+ 1 x)) (vector->list wv))) |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
parents:
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(display "OK\n") |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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Interesting documents from Oleg about monads and exceptions
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And it is still OK. We can even print what we've got: |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(display (if-error (car wl) (lambda () "error"))) |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(newline) |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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Interesting documents from Oleg about monads and exceptions
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which prints the expected result. It's only |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(display (if-error (cadr wl) (lambda () "error"))) |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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(newline) |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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that prints an error. The examples tested on Bigloo 2.4b and Gambit 3.0. |
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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Interesting documents from Oleg about monads and exceptions
Tony Garnock-Jones <tonyg@lshift.net>
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Thus we can have our exceptions, and deal with them lexically, too. |
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Interesting documents from Oleg about monads and exceptions
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Interesting documents from Oleg about monads and exceptions
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