ClauseDifferences from Erlang/OTP
Observable behavior where Clause knowingly differs from the pinned OTP 29
(maint-29, see otp-reference.md). Each entry links to the
contract that owns it. Remove an entry once the difference is fixed. Features
not implemented yet (reported as notimpl) are not listed here; see
features.md and the plan.
Maps
| Difference | OTP | Clause | Owner |
|---|---|---|---|
Map generator order (K := V <- M) | Flat maps (up to 32 keys) iterate in key order, but atom keys in atom-table order, which varies between VM runs; larger maps in hash order | Always canonical key order (term order) | patterns |
bad_generators payload of a map generator in a zip group | Its iterator: {K, V, Next} chain ending in none, in OTP's order | The same chain, built in canonical key order | patterns |
Map printing (erlang:display/1, ~w, ~p) | Internal layout order (atom-table order for atom keys, hash order above 32 keys) | Map-key order, as OTP's ~kw | terms |
Native record printing (erlang:display/1) | Fields in atom-table index order | Definition order, as ~w | native records |
== between native records whose fields differ only as integer/float | The compiler may fold it to =:= (false) | Numeric comparison (true) | native records |
Local fun printing (#Fun<M.Index.Uniq>) | Index from the compiler's lambda numbering, Uniq a hash of the module code | Index of the module's local funs in source order, Uniq always 0 | funs |
| Order of two local funs of one module | By OTP's index | By source-order index, so funs of different functions can order differently | funs |
Anonymous fun names in stack traces (-f/1-fun-N-) | N from the compiler's numbering | N counts the funs of f/1 in source order | funs |
| An anonymous fun in a record field default | Each construction site expands its own copy: funs from two sites are unequal | One fun for the default: funs from any construction site are equal | funs |
| Captured value order of funs created inside comprehensions | Free variables of the comprehension's generated function | Definition order in the enclosing function; visible only when comparing two such funs | funs |
Pids, references and processes
| Difference | OTP | Clause | Owner |
|---|---|---|---|
| Pid numbers | The first user process is about <0.80.0>; numbers are reused after the pid table wraps | The first process is <0.1.0>; numbers come from one sequence and are never reused | terms |
Code after spawn(Fun) of a fun of another arity | The compiler's type analysis may treat the code after the call as unreachable and drop it, so the caller returns the pid at once | The caller goes on; only the new process fails with {badarity, {Fun, []}} | processes |
Formatting a large term (io:format/1,2) | Runs in Erlang code (io_lib) and the group leader, so the process can be preempted while formatting | Runs to completion in one builtin call; other processes wait | builtins |
process_flag/2 flags | trap_exit, priority, message_queue_data, min_heap_size and others | Only trap_exit; every other flag raises badarg | processes |
| Error report order of processes crashing at the same time | Logger order | The order their workers finish them | processes |
| Signals to a process running on another scheduler | Queued in its signal queue and handled later, while the sender goes on | The sender waits until the target's time slice ends, then acts at once; observable only as timing | processes |
list_to_port/1 of a number never issued | Returns a port term | badarg: only ports this program opened are admitted | ports |
port_info/1,2 values | id is a table index; memory, queue_size, locking describe the driver | id is the port number; memory and queue_size are 0; locking is port_level | ports |
| Writing a file opened only for reading | {error, eacces} on Windows, {error, ebadf} elsewhere | {error, ebadf} everywhere | ports |
io:get_line, io:get_chars and file:read/2 in list mode on non-ASCII input | Characters decoded per the device encoding | One list element per byte | ports |
os:cmd/1 result | Unicode characters decoded from the output | The output's bytes | ports |
Empty arguments of {spawn_executable, F} on Windows | Dropped from the command line | Passed as "" | ports |
{exit_status, S} and eof of a spawned program | Unspecified order, exit_status may arrive before the last data | After all data, before eof | ports |
| Busy ports | A port with too much queued output suspends senders; port_command/3 force works on drivers that allow it | Output never suspends; force raises notsup on every driver | ports |
{active, N} of sockets | An integer N counts messages before {tcp_passive, S} | exit(badarg); only true, false and once | ports |
Socket tuning options (nodelay, keepalive, send_timeout, delay_send, buffers) | Applied; other inet options (header, {packet, line}, http, ...) work | The listed tuning options are accepted and not applied; other options are exit(badarg) | ports |
| Socket error reasons | Every POSIX reason the system reports | The common ones (econnrefused, eaddrinuse, econnreset, etimedout, ...); others are eio | ports |
| Port input while the connected process holds 1,024 messages and can run | Keeps reading and delivering | Stops delivering until the process's time slice ends; past 64 KiB held, stops reading, so the writing program blocks | ports |
gen_tcp:close/1 with queued output | Waits up to the linger/send_timeout setting for the output to leave | Returns at once; the I/O thread sends the queued output, then closes | ports |
monitor/2 types | process, port, time_offset | process and port; time_offset raises badarg (no time offset changes) | processes |
exit/2, exit_signal/2 to a reference | Sends to the process alias, if the reference is an active one | Nothing (no aliases) | processes |
| Reference numbers | Mix a scheduler identifier and per-scheduler counters (#Ref<0.178111994.4235460610.214105>) | One program-wide counter (#Ref<0.0.0.1>), so references order by creation | terms |
Errors, stack traces and reports
| Difference | OTP | Clause | Owner |
|---|---|---|---|
| Stack trace locations | [{file, F}, {line, L}], error_info for error/3 | Always []; no error_info | ABI |
function_clause top frame | Argument list | Arity | ABI |
undef top frame | {M, F, Args, []} of the missing function | The calling function's frame | funs |
| Failing BIF/operator frames | Present ({erlang, '+', Args, ...}) | Absent; nothing below the entry function | ABI |
Atom table full (list_to_atom/1) | The emulator aborts (no more index entries in atom_tab) and writes a crash dump | clau: runtime failure: entry call failed: resource_limit, exit 70 | builtins |
list_to_integer/1,2 with characters above 255 | Its first digits use only each character's low byte ([16#131] is 1) | badarg | builtins |
erlang:function_exported/3 of a BIF | True for every BIF of the emulator | True only for the builtins this runtime provides | builtins |
| Error reports of crashed processes | Sent to the logger, written later by its default handler (on standard output under erl, often lost when an escript halts first) | Written on stderr when the process ends, before any later output | processes |
| Calls to functions that never return | Compiled as tail calls (caller missing from the trace) | Ordinary calls (caller present) | ABI |
| Host memory exhausted | The emulator reports that it cannot allocate memory, writes a crash dump and stops | clau: runtime failure: entry call failed: out_of_memory, exit 70, no dump | runtime heap |
| Process memory cap | max_heap_size (in words, heap and stack) kills the process with reason killed and logs an error report | --max-heap / --max-stack (bytes) and the runtime-wide --max-memory fail the requesting process as resource_limit: clau: runtime failure: entry call failed: resource_limit, exit 70 | runtime heap |
| Program arguments | escript passes every argument to main/1; emulator flags (+t) come from %%! or ERL_FLAGS | Leading runtime options (--max-atoms, --max-heap, --max-stack, --max-memory, --args-file, --) are taken out first; CLAUSE_FLAGS holds the same options | executables |
| Uncaught exception in an ordinary entry module | escript exits 127 | Exits 1 with one uncaught exception <class>: <reason> line (escript sources keep 127) | executables |
| Compiler diagnostics | erl_lint wording (variable 'X' is unbound) and warnings | Own wording (unbound variable X); OTP lint warnings are mostly not emitted (also unknown native record fields in access, update and patterns) | semantic |
Language edge cases
| Difference | OTP | Clause | Owner |
|---|---|---|---|
Integer segment wider than the integer limit, value past it (<<V:4194241>> of all ones) | The x86 JIT matches with an invalid term; using it crashes the VM | No match | terms |
| Zip group whose relaxed and strict generators share a variable | Skip test keeps strict-pattern variables in the relaxed patterns | A rejected step is skipped whenever the strict patterns match on their own | patterns |
Calling an external fun, M:F(Args) or apply/3 of a module outside the program | Loads the module from the code path, undef when absent | undef; library modules join the program only when it names them with a literal atom | funs, library |
lists and maps | The full modules; maps:keys/1, values/1, to_list/1, fold/3 follow the map's internal order (atom-table order for atom keys, hash order above 32 keys) | The library subset; map functions follow key order | library |
Files without #! given as escripts | escript file.erl skips the first line | Compiled as ordinary modules | executables |
| Precompiled beam and archive escripts | Run | Not supported | executables |
A -spec whose types share no value with the inferred result, entry domain or a call's arguments | Compiles; Dialyzer may warn | Compile error naming the declared and inferred types | semantic analysis |
io
| Difference | OTP | Clause | Owner |
|---|---|---|---|
Control sequences ~e ~f ~g ~x ~X ~+ ~# ~W ~P, modifier K | Formatted | badarg | io |
~p of containers nested more than 256 deep | Printed | system_limit | io |
Field widths and ~ts precision with combining characters or \r\n | Count grapheme clusters | Count code points | io |
Negative precision or pad count (~.*c with -1) | Loops forever | badarg | io |
io functions other than format/1,2, put_chars/1 | Exist | unknown module io | io |