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> Errors as return values is only acceptable for code that is so performance-sensitive that you aren't allowed to do dynamic memory allocations Not really. It

• b3morales 4y ago
  It's probably sufficient for this conversation to just understand it as try-catch. A function is invoked; if it "signals" (throws) then control moves to a handler that matches the signal (exception); the handler runs and resolves the situation. Of course, Lisp being Lisp, the system is extended to announcer voice FULL. GENERALITY. but in its simplest form it's basically equivalent to exception throwing.
  • ThatGeoGuy 4y ago
    This is correct (in that the most simplistic case is try-catch). However, the difference between a try-catch and conditions / restarts is that when one signals a condition (exception), the restart (catch) has a continuation from the condition. This allows you to inject an expression into the location where an exception occurred and "restart" your code from that point. Whether you do such a thing or not depends on the code, on the type of condition raised, and on what expressions are valid. So you get a lot more flexibility in how errors are handled across the system. But likewise: more complexity in having to make that choice in the first place. Going farther than this, conditions and restarts are really just a fancy way of packaging delimited continuations. I don't personally know any non-Lisp language that has attempted to package these concepts (maybe Dylan, which is a Lisp-like in its own way but without the syntax?). Going back to the original thought regarding error handling - I think Result<T, Err> type handling is fine and that most languages would be better served by that than having different types of exceptions. Conditions and restarts are powerful but your language has to be very expression focused (i.e. does not use a lot of statements) and it's not really clear that there's been a lot of work on making restarts nice to use. Exceptions in all languages that have them have their own set of associated problems, for what its worth, and it's not as easy to move Lisp features into a non-Lisp as one might believe...
    • mananaysiempre 4y ago
      > Going farther than this, conditions and restarts are really just a fancy way of packaging delimited continuations. I don't personally know any non-Lisp language that has attempted to package these concepts (maybe Dylan, which is a Lisp-like in its own way but without the syntax?). Dylan does have a condition system, but it’s basically a Lisp without the parens, so probably doesn’t count. On the other hand, algebraic effects are another fancy way of packaging delimited continuations, so arguably the research languages Eff[1] and Koka[2] tried. (I don’t think either one explored the connection with condition systems, but I’m not sure.) > I think Result<T, Err> type handling is fine and that most languages would be better served by that than having different types of exceptions. Conditions and restarts are powerful but your language has to be very expression focused (i.e. does not use a lot of statements) [...] Huh? I don’t know why you’d say that, if anything I think it’s the Either err t / Result<T, Err> style that is more expression-focused (I mean, it even originates in Haskell :). I wouldn’t even call Common Lisp particularly expression-oriented, honestly, not unless we’re comparing with plain old C and not Rust. [1] https://www.eff-lang.org/ https://www.eff-lang.org/ [2] https://koka-lang.github.io/ https://koka-lang.github.io/
      • ThatGeoGuy 4y ago
        > Huh? I don’t know why you’d say that, if anything I think it’s the Either err t / Result<T, Err> style that is more expression-focused (I mean, it even originates in Haskell :). I wouldn’t even call Common Lisp particularly expression-oriented, honestly, not unless we’re comparing with plain old C and not Rust. I think that's exactly what I mean. The vast majority of languages (including golang, in TFA) use statements for dealing with exceptions. Rust also had try-catch, but has long since removed that syntax. Anyways, the reason I said it is because it is not clear what to do when one wants to restart a statement. There are plenty of non-expressions that can throw, and usually it's not thought about deeply, but from a language semantics point of view one does need to have an idea of how to engage with it. For example, if you wrote: with open('somefile') as f: for line in f: # ... in Python, and had to deal with a restart during `open`, how do you manage this? The naive answer is to just return the continuation at `open`, but the "with" statement may have contextual setup. For example, `open` might be fine during `__init__`, but may have failed in `__enter__`. If you "restart" in `__enter__`, you need to deal with the partial state. Expression-based languages don't really have this issue because the call stack is usually clear (there's no magic under the hood). Similar analogues would be the `using` keyword in C#, or perhaps even lambda-expressions in C++. The abstraction in the code is separated from the execution of the restart, so it gets kind of gross as a language implementer in terms of not having to have very specific places where restarts can and cannot be. This is a good reason why Rust / Haskell don't package these and just use Either / Result instead. If you have a bunch of types that you didn't write, injecting a restart into any failing code now brings a question of: "Can you safely inject types into a restart for code that you do not have access to?" and the answer is often no. The visibility rules in Rust make this a non-starter, and in Haskell you have a problem of mutability as well. A condition may be triggered at a point where IO could be injected, and so many of the language semantics would be in question. I suspect the type definitions for a restart in any arbitrary location in the code would be pretty hard to write, so maybe this is an open research area in Haskell already, but I doubt it'd be as ergonomic.
• mananaysiempre 4y ago
  > I don't really know what "conditions+restarts" is but a few articles landed me into LISP which I find totally unreadable. So, can you point me to some "conditions+restarts" code that I can understand/appreciate easily? You’ll have to read Lisp, I’m afraid; the best description I know is in the book Practical Common Lisp[1]. (Come on, Lisp syntax is quirky, but it’s not unreadable, and unlike APL or Forth or even Haskell it doesn’t require you to memorize a bunch of semi-meaningless punctuation before you can understand what is going on—it’s pretty wordy usually. I’m not saying you must bring yourself to love writing (f x y) instead of f(x, y), only that adjusting from one to the other should not be particularly hard.) I mean, I have done a toy Forth implementation, but that is hardly more readable with no experience with the language. One system that is almost conditions and restarts is 32-bit(!) Win32 SEH, but it is not particularly well-documented and the language bindings usually try rather hard to hide that (though, if you think about it, On Error Resume Next from classic VB is unimplementable on top of bare try/catch). ... OK, you nerd-sniped me :) Here’s a toy (no subtyping! no introspection! no condition firewall[2]! no tracebacks! no support for native errors! etc.) condition system in Lua (sorry, nested functions in Python are painful): -- save as cond.lua local M = {} local error, unpack = error, unpack or table.unpack local running = coroutine.running local stderr = io.stderr local exit = os.exit local insert, remove = table.insert, table.remove -- conditions local handlers = setmetatable({}, { __mode = 'k', -- do not retain dead coroutines __index = function (self, key) -- no handlers by default self[key] = {}; return self[key] end, }) local function removing(xs, x, ok, ...) assert(remove(xs) == x) if ok then return ... else error(...) end end -- establish a handler during call function M.hcall(h, f, ...) local hs = handlers[running()] insert(hs, h) return removing(hs, h, pcall(f, ...)) end -- signal the given condition to currently active handlers function M.signal(...) local hs = handlers[running()] for i = #hs, 1, -1 do hs[i](...) end end local signal = M.signal function M.error(...) signal(...) stderr:write("error: " .. tostring(...) .. "\n") exit(1) end function M.warn(...) signal(...) stderr:write("warning: " .. tostring(...) .. "\n") end -- restarts -- invoke the given restart function M.restart(r, ...) local n = select('#', ...); r.n = n for i = 1, n do r[i] = select(i, ...) end error(r) end local function continue(r, ok, ...) if ok then return ok, ... end if ... == r then return false, unpack(r, 1, r.n) end error(...) end -- establish a restart during call function M.rcall(f, ...) local r = {} return continue(r, pcall(f, r, ...)) end return M Example: DOS-style abort-retry-ignore prompt implemented in the shell with some support in the (mock) I/O system and no support in the application: local cond = require 'cond' -- common condition types (XXX should use proper dynamic variables instead) local retry, use = nil, nil -- I/O library local function _gets() if math.random() < 0.5 then cond.error 'lossage' end return 'user input' end local function gets() local ok, value = cond.rcall(function (_use) use = _use local ok, value repeat ok, value = cond.rcall(function (_retry) retry = _retry return _gets() end) until ok return value end) -- ok or not, we got a value either way return value end -- application (knows nothing about errors) local function app() for i = 1, 5 do print(string.format("got: %q", gets())) end return "success" end -- shell local ok, value = cond.rcall(function (abort) return cond.hcall(function (err) io.stderr:write("I/O error: " .. err .. "\n") while true do io.stderr:write("[a]bort, [r]etry, [u]se value? ") local answer = io.read('*l') if answer == 'a' then cond.restart(abort, "aborted") end if answer == 'r' then cond.restart(retry) end if answer == 'u' then io.stderr:write("value? ") cond.restart(use, io.read('*l')) end end end, app) end) print(ok, value) This is not a perfectly accurate semantic model for real condition system, but it should be enough to give a general idea of how these things work and what the advantage over bare unwinding mechanisms like try / throw or Lua’s pcall / error is. [1] https://gigamonkeys.com/book/beyond-exception-handling-conditions-and-restarts.html https://gigamonkeys.com/book/beyond-exception-handling-condi... [2] https://www.nhplace.com/kent/Papers/Condition-Handling-2001.html https://www.nhplace.com/kent/Papers/Condition-Handling-2001....
• throw10920 4y ago
  > It is acceptable for code whose maintainers value readability and simplicity over everything else. Errors-as-return values are less readable than conditions, not more - there's literally more visual noise on the screen. And if you want "simplicity", don't use a computer. Computers are intrinsically complex devices, users desire features with complex implementations, and our job as programmers is to manage complexity, not pretend that it doesn't exist. One of the article's main points is that Go does the latter in lieu of the former, and that's also what errors-as-return-values does. ---------------------------------- The formal name for a condition+restart system appears to be "algebraic effects"[1]. Conditions and restarts are similar to exceptions, with the following changes: First, conditions are conceptually used for non-error conditions in some cases, like what Python does. Second, throwing a condition doesn't cause the stack to unwind up to the handler, unlike exceptions. Third, in addition to throwing conditions, you, uh, wrap ("establish" is the jargon used) code in what are called "restarts", similar to wrapping things in try/catch blocks (but distinct, because with conditions you still have condition handling blocks). Restarts can have names and are non-mutually-exclusive. Conceptually, restarts represent error-recovery strategies, while conditions represent the errors themselves. Fourth, when a condition is thrown, it propagates upward until it hits either the toplevel (in which case the interactive debugger is launched), or it hits a condition handler - without unwinding the stack. Then, either the human looking at the debugger can pick which restart they want to use, or the logic at the condition handler can do so. Why is this better than any alternative error-handling mechanism? Because every other error-handling mechanism (1) unwinds the stack (destroying all contextually useful information that isn't explicitly saved by the programmer, and preventing you from restarting a computation in the middle) (2) forces you into a single error-recovery strategy and (3) couples low-level code to high-level code as a result. In general, low-level code has details about the specific kind of error, context around it, and access to data and control flow that would allow the error to be recovered from (e.g. for a log-processing program, reasonable restarts while parsing a log entry would be (1) skip it (2) retry (3) use an alternative parser and (4) return an empty entry), while high-level code has the application context about why the low-level operation is being performed in the first place and which error-recovery option should be picked. Conversely, high-level code doesn't have details about what the low-level code was doing at the time of the error, and low-level code doesn't have the high-level context necessary to determine which error recovery strategy is appropriate in this use of the low-level code. [1] https://en.wikipedia.org/wiki/Effect_system https://en.wikipedia.org/wiki/Effect_system https://en.wikipedia.org/wiki/Exception_handling#Condition_systems https://en.wikipedia.org/wiki/Exception_handling#Condition_s...