output writes to files and stdout. log writes to stderr. append adds to existing files. stream enables real-time token-by-token output from LLM calls.

Output Directive

output @content to "out.txt"
output @data to "config.json"
output @message to stdout
output @error to stderr
output @config.yaml() to "settings.yaml"
output @results.json() to "results.json"
output @plain.text() to "plain.txt"

Use transformer methods before output when you need a specific file format:

Transformer	Behavior
@value.json()	Emit JSON text
@value.yaml()	Emit YAML text
@value.text()	Emit plain text

Examples:

output @data.json() to "results.json"
output @data.yaml() to "config.yml"
output @data.text() to "plain.txt"

See Also

• Append Directive - Append structured or text output incrementally.
• Log Directive - Emit output to stderr.
• Pipelines Basics - Attach output as a pipeline effect stage.

Log Directive

var @debugMode = true

log @message                        >> same as output @message to stderr
log `Processing: @item`

>> In action contexts
for @item in @items => log @item
when @debugMode => log "Debug info"

See Also

• Output Directive - File/stdout/stderr output patterns and formatting.
• Pipelines Basics - Use | log as a pipeline effect stage.

Append Directive

append @record to "events.jsonl"        >> JSON object per line
append "raw line" to "events.log"

>> In pipelines
var @_ = @data | append "audit.jsonl"

>> In loops
for @name in @runs => append @name to "pipeline.log"

.jsonl enforces JSON serialization. Other extensions write text. .json blocked.

Streaming

stream @claude("prompt")           >> keyword form
stream @generateReport()           >> directive form

>> Parallel streams
stream @a() || stream @b()         >> concurrent, buffered results

Suppress: --no-stream flag or MLLD_NO_STREAM=true