00-sample.json

{ "path": "path/to/Main'.elm", "line": 1, "column": 1, "message": "<file would change>"}
{ "path": "path/to/Main'.elm", "line": 2, "column": 1, "message": "file would change"}
{ "path": "path/to/View.elm", "line": 1, "column": 1, "message": "file would change"}

01-output-sample.txt

# sample json contains XML special chars to check if they get correctly encoded

$ cat 00-sample.json | jq -n -r -f checkstyle.jq
<?xml version="1.0" encoding="utf-8"?>
<checkstyle version="5.7">
<file name="path/to/Main&apos;.elm">
<error line="1" column="1" severity="error" message="&lt;file would change&gt;" source="elm-format">
<error line="2" column="1" severity="error" message="file would change" source="elm-format">
</file>
<file name="path/to/View.elm">
<error line="1" column="1" severity="error" message="file would change" source="elm-format">
</file>
</checkstyle>

$ cat 00-sample.json | jq -r -f pep8.jq
path/to/Main'.elm:1:1: <file would change>
path/to/Main'.elm:2:1: file would change
path/to/View.elm:1:1: file would change

checkstyle.jq

def preamble:
    # This format is based on:
    # https://github.com/checkstyle/checkstyle/blob/release5_7/src/checkstyle/com/puppycrawl/tools/checkstyle/XMLLogger.java
    "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<checkstyle version=\"5.7\">\n";

def postamble:
    "\n</checkstyle>";

def map_join(f):
    map(f) | join("\n");

def format_error:
    @html "<error line=\"\(.line)\" column=\"\(.column)\" severity=\"error\" message=\"\(.message)\" source=\"elm-format\">";

def format_file:
    @html "<file name=\"\(.key)\">\n" + (.value | map_join(format_error)) + "\n</file>";

reduce inputs as $error
({};
 $error.path as $path
 | has($path) as $newFile
 | (if $newFile then .[$path] else [] end) as $prevErrors
 | . + { ($path): ($prevErrors + [$error]) })
| preamble + (to_entries | map_join(format_file)) + postamble

pep8.jq

# We might need an error code before the message:
# haven't tried against a real pep8 output parser
"\(.path):\(.line):\(.column): \(.message)"