Suggest a command or flag when one is mistyped - #17
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A mistyped command used to produce "error parsing flags: unexpected
arguments: [update]". Nothing about flags was wrong, the Go slice syntax
leaked into user-facing output, and the dispatcher knew the real command
names all along without offering any of them.
Unknown commands now report the offending word and, when something is
close enough, what the user probably meant:
unknown command "update" for "miren app"
Did you mean?
upgrade
Run 'miren app --help' to see available commands.
Three paths fed into the old behavior, and all three are covered:
- A word matching nothing at all. This previously joined every argument
into the message, so "miren depoy myapp" blamed "depoy myapp". It now
walks the typed words to find the deepest valid namespace and blames
only the first word past it.
- A leftover word under a command that has sub-commands.
- A leftover word under a command that tolerates unknown flags. These
skip the extra-argument check entirely, so a typo used to print help
and exit 0 with no hint that anything was wrong.
The last two share one rule: a leftover word under a command that has
children and declares neither positionals nor a rest field can only be a
misspelled sub-command. The check runs after Parse, so that a word which
is really an unknown flag's value is claimed as such rather than blamed.
Suggestions come from a Levenshtein distance over runes with a threshold
that scales with word length, plus a prefix rule so "depl" reaches
"deploy". Written by hand to keep this module free of dependencies.
Errors are now typed. UnknownCommandError carries the program, parent
path, bad word, and candidates so callers can render it themselves;
UnexpectedArgsError replaces the "%v" slice formatting and no longer gets
wrapped in "error parsing flags", which is reserved for real flag errors.
Extends the command suggestions to flags, which fail the same way and
were just as silent about it:
unknown flag: --naem
Did you mean?
--name
Only long flags get a guess. A single letter is too little to reason
from, and offering long flags that merely start with it would be noise.
Two supporting changes:
Edit distance now counts a transposition of adjacent characters as one
edit rather than two. Swapping two letters is among the most common ways
to mistype a word, and the old cost put "naem" two edits from "name" —
outside the threshold for a word that short, so the obvious suggestion
was never offered. This improves the command suggestions too.
UnknownFlagError is no longer wrapped in "error parsing flags". The
message already says a flag is the problem, and the prefix only pushed
the suggestions further from the summary. Genuine value errors, such as
a non-numeric argument to --port, keep it. The error still unwraps to
ErrUnknownFlag, so existing errors.Is checks are unaffected.
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Included review availability: 3 reviews are currently available. Your included PR review attempts over the past 7 days set your current allowance at 8 reviews per hour. 📝 WalkthroughWalkthroughThe change adds typed errors for unknown commands, unknown flags, and unexpected arguments. It adds rune-aware edit-distance suggestions with deterministic ranking and a three-result limit. Dispatcher parsing now reports namespace-aware command errors and preserves specific flag and argument errors. Tests cover suggestions, namespaces, valid arguments, error unwrapping, and exact message formatting. Priority: ➖ Normal Merge Risk: ⚪ Minimal · up to The command-error reporting changes have no identified merge-blocking risk. Comment |
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🍪 biscuit: ✅ ready to merge — auto-review, non-blocking
I read through the full diff and the core new files carefully. The feature — "did you mean?" suggestions for mistyped commands and flags — is well-designed and well-tested. Here's what I found.
What the PR does
It introduces a Damerau-Levenshtein edit-distance function in suggest.go, wires it through three new error types in errors.go (UnknownCommandError, UnknownFlagError, UnexpectedArgsError), and updates dispatcher.go and mflags.go to generate and return those errors instead of plain fmt.Errorf strings.
Correctness
The rolling-row edit-distance implementation in suggest.go is correct — the prevPrev/prev/curr rotation is the standard Damerau-Levenshtein approach. The threshold table (suggestThreshold) is conservative and sensible: no distance-based guessing for 1–2 character inputs, capping out at 3 for longer words. Prefix matching bypasses the threshold intentionally, and the comment explains why. I traced the unknownCommandError → unknownSubCommandError → suggestNames path and confirmed it correctly finds the deepest valid namespace before blaming a word.
Edge cases handled
The test suite in unknown_command_test.go covers the scenarios that matter most: prefix typos, sub-command typos, section-style commands with unknown flags, commands with positional args (which must not fire), rest-field commands (also must not fire), the help keyword, and the case where no close match exists. The TestUnknownCommandDoesNotFireOnValidInput table is a particularly good regression guard.
TestUnexpectedArgsWithoutSubCommands confirms that UnexpectedArgsError is returned unwrapped (no "error parsing flags:" prefix) even when there are no sub-commands to suggest.
The mistypedSubCommand guard — checking fs.PositionalCount() > 0 || fs.restField != nil before blaming an unknown word — is the right place to avoid false positives.
Breaking changes
The old unknown command: foo bar string is gone from the dispatcher; callers who relied on strings.Contains(err.Error(), "unknown command:") will now see unknown command "foo" format. That's a semantically better error, but it is a message-format change. The new UnknownCommandError and UnknownFlagError types give callers a structured alternative, which is a net improvement for programmatic use. ErrUnknownFlag remains unwrapped through UnknownFlagError.Unwrap(), so errors.Is callers are unaffected. This is the right call.
One small observation — not a blocker, but worth noting: unknownLongFlagError in mflags.go compares the typed name (stripped of --) against all candidate names (also stripped), which is correct. But suggestNames for flags is called only for long flags; short flags intentionally get no suggestions (the comment explains: a single letter is too little to guess from). That's a deliberate design choice and is tested.
Overall this is clean, thoroughly considered work. I'm happy to see it merged.
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Inline comments:
In `@dispatcher.go`:
- Line 299: Update the entry == nil branch in Dispatcher.Execute to detect when
the original input contains only flag tokens and return an UnknownFlagError
identifying the unknown flag, instead of passing an empty nonFlagArgs slice to
unknownCommandError. Preserve the existing unknownCommandError behavior for
inputs that include non-flag arguments.
In `@mflags_test.go`:
- Line 1095: Update the seven UnexpectedArgsError assertions in FlagSet.Parse
tests from assert.Contains to assert.EqualError, preserving each existing
expected error message exactly and enforcing the direct, unwrapped parser error
contract.
In `@mflags.go`:
- Around line 715-716: Update the candidate-building loop in
unknownLongFlagError to exclude entries whose Flag.Hidden marker is true, while
retaining visible flags for suggestions and keeping hidden flags parseable
through parseLongFlag.
In `@suggest.go`:
- Line 109: Update the prefix match distance calculation in the scored append
logic to use rune counts for both c and unknown instead of byte lengths, while
preserving the existing distance semantics and prefix ranking behavior.
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Three fixes from review on #17. Flag-only input reported `unknown command ""`. Execute filters flag tokens out before building the error, so `miren --bogus` had no word left to name — a regression against the old `unknown command: --bogus`. It now reports the flag instead, without suggestions, since no command matched and there is no flag set to draw candidates from. `--bogus=value` reports just `--bogus`. Hidden flags no longer appear in suggestions. help.go already keeps them out of help output, and offering one on a near-miss typo would undo that. Prefix completions rank by characters rather than bytes, so a short non-ASCII completion is no longer sorted behind a longer ASCII one. The rest of the matching was already rune-based; this line was the holdout.
Picks up three fixes from review on mirendev/mflags#17: flag-only input such as `miren --bogus` now names the flag instead of reporting an empty command name, hidden flags stay out of suggestions, and prefix completions rank by characters rather than bytes. Adds the flag-only case here too, since it is user-visible and was a regression against the previous release.
mirendev/mflags#17 is merged; repoint from the branch commit to main. No behavior change — same code, now reachable from mflags main.
2 participants
A mistyped command used to produce this:
Nothing about flags was wrong, the Go slice syntax leaked into user-facing
output, and the dispatcher knew every valid command name all along without
offering any of them.
Now:
Three paths fed the old behavior
Depending on what the dispatcher managed to match, a typo failed in one of
three different ways. All three are covered:
miren depoyunknown command: depoymiren app lsterror parsing flags: unexpected arguments: [lst]miren runner updateThe third was the quiet one: commands that tolerate unknown flags skip the
extra-argument check entirely, so a typo looked like success.
The last two share a rule: a leftover word under a command that has children
and declares neither positionals nor a rest field can only be a misspelled
sub-command. The check runs after
Parse, so a word that is really anunknown flag's value gets claimed as one rather than blamed — that is what
keeps
myapp -C prod authworking, and it is the reason the check cannot liveinside
Parseitself.Two smaller repairs along the way.
unknown command: depoy myappused toblame every word typed; it now walks the words to find the deepest valid
namespace and blames only the first word past it. And
unexpected arguments: [foo]no longer prints a Go slice.Matching
Edit distance over runes, with a threshold that scales with word length, plus
a prefix rule so
deplreachesdeploy. A transposition of adjacentcharacters costs one edit rather than two — swapping two letters is among the
most common ways to mistype a word, and the plain-Levenshtein cost put
naemtwo edits from
name, outside the threshold for a word that short.Written by hand to keep this module free of dependencies.
Against the real miren command tree, every realistic typo I tried lands on
exactly one correct suggestion (
lst→list,wohami→whoami,upgrde→upgrade,verisons→versions,rollbak→rollback). Nonsensegets no guess at all. That silence is deliberate: a confident wrong suggestion
is worse than none, so the thresholds err toward staying quiet.
Only long flags get a guess. A single letter is too little to reason from, and
offering every long flag that merely starts with it would be noise.
Errors are now typed
UnknownCommandErrorcarries the program, parent path, bad word, andcandidates, so callers can render it themselves instead of parsing the message
back out of a string.
UnexpectedArgsErrorreplaces the%vsliceformatting.
UnknownFlagErrorstill unwraps toErrUnknownFlag, so existingerrors.Ischecks are unaffected.Neither
UnexpectedArgsErrornorUnknownFlagErroris wrapped in "errorparsing flags" any more — leftover arguments are not a flag problem, and
"unknown flag" already says which part of the command line went wrong. Genuine
value errors, such as a non-numeric argument to
--port, keep the prefix.Behavior change
miren runner updateand its shape go from exit 0 to exit 1. Anythingdepending on that was already depending on a typo, but it is a real change.
Testing
go test ./...passes. New coverage insuggest_test.goandunknown_command_test.gofor each error path, plus the inputs the check mustleave alone: help keywords, pass-through arguments, and value-taking global
flags before a section name. Existing assertions on the old wording are
updated.
Also verified against a real
mirenbuild across the full command tree — boththat every typo shape reports well, and that valid invocations (
app run echo hi,-C prod app list,auth help,app --help) are untouched.MIR-1823