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Lets an agent type-check a DSL expression before spending a full input
pass. `mlr put --explain '...'` (and filter) runs the existing
parse -> ValidateAST -> CST build -> Resolve path, then:
- valid: prints "mlr {put,filter}: DSL expression is valid." and exits 0
- invalid: returns the build error up the normal path, so --errors-json
emits a structured document; exits 1
- -W with fatal warnings: reports and exits 1
The gate lives in the pass-two constructor, before any input file is
opened, so no input stream is read (verified with a nonexistent input
file still validating OK).
Also categorize bare "parse error: ..." messages from the DSL parser as
kind "dsl-parse-error" rather than "generic" (climain/errors_json.go),
so --explain --errors-json gives an agent a useful error kind. The CSV
reader's "parse error on line ..." is stream-time and never reaches this
command-line-parse categorizer.
Tests: dsl-explain/0001-0004 regression cases (valid put/filter, invalid
plain, invalid --errors-json) and categorize unit tests. Regenerated
verb docs, manpage, and the help usage-verbs golden case.
The older -X ("exit after parsing") still exits 0 even on a parse error;
left as-is since --explain is the correct validation path.
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
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