Export Directives
Export directives are used to write computation results to files.
Syntax
The syntax of an export directive is as follows:
@export <predicate> :- <format>{<parameter>=<value>, ...} .For example, the following export writes a CSV file that contains two columns, with the first formatted as plain string and the second as an integer. The third parameter of the exported predicate result is skipped and will not be in the file:
@export result :- csv{resource="out.csv.gz", format=(string,int,skip)} .The output will be gzipped (guessed from file name). Tuples that contain anything other than a string in the first component and an integer in the second will not be exported and ignored silently.
Formats
Currently the following formats are supported:
| Format | Description |
|---|---|
csv | Comma-separated values. |
dsv | Delimited separated values. (Like csv, but allows specifying different delimitors) |
rdf | RDF turtle format. |
Parameters
The following parameters are available for all formats:
| Parameter | Description |
|---|---|
resource | The file name to write to. If it contains an extension, this is used to automatically set the gzip parameter. |
Instead of writing data to a file, one can also return it in the standard output (on the command line). For this, an empty string should be given as a resource, as in this example that prints the first 10 triples in RDF format:
@export triples :- turtle{resource="", limit=10} .In exports, it is also possible to omit the resource parameter altogether. In this case, a default file name will be chosen based on the exported predicate, file format, and compression. For example, the following will export to triples.nt.gz:
@export triples :- ntriples{compression="gzip"} .When using the Nemo command-line client, some options are available to override the export directives in the program, to set the output (base) directory, and to control if existing files should be overwritten.