Add empty syntax type for suppressing output

7 files changed, 70 insertions, 17 deletions

diff --git a/doc/serdi.1 b/doc/serdi.1
index 22ca352f..7cb2c923 100644
--- a/doc/serdi.1
+++ b/doc/serdi.1
@@ -67,6 +67,7 @@ Case is ignored, valid values are:
 .Dq NQuads ,
 .Dq NTriples ,
 .Dq TriG ,
+and
 .Dq Turtle .
 .Pp
 .It Fl k Ar bytes
@@ -86,10 +87,15 @@ Note that data may be lost when using this option.
 Write output as
 .Ar syntax .
 Case is ignored, valid values are:
+.Dq empty ,
 .Dq NQuads ,
 .Dq NTriples ,
 .Dq TriG ,
+and
 .Dq Turtle .
+When
+.Dq empty
+is given, output is suppressed, so only errors will be printed.
 .Pp
 .It Fl p Ar prefix
 Add
diff --git a/include/serd/serd.h b/include/serd/serd.h
index ba1b4ddc..79353e50 100644
--- a/include/serd/serd.h
+++ b/include/serd/serd.h
@@ -127,10 +127,11 @@ typedef enum {
 
 /// RDF syntax type
 typedef enum {
-  SERD_TURTLE   = 1, ///< Terse triples http://www.w3.org/TR/turtle
-  SERD_NTRIPLES = 2, ///< Line-based triples http://www.w3.org/TR/n-triples/
-  SERD_NQUADS   = 3, ///< Line-based quads http://www.w3.org/TR/n-quads/
-  SERD_TRIG     = 4  ///< Terse quads http://www.w3.org/TR/trig/
+  SERD_SYNTAX_EMPTY = 0, ///< Empty syntax (suppress input or output)
+  SERD_TURTLE       = 1, ///< Terse triples http://www.w3.org/TR/turtle
+  SERD_NTRIPLES     = 2, ///< Flat triples http://www.w3.org/TR/n-triples/
+  SERD_NQUADS       = 3, ///< Flat quads http://www.w3.org/TR/n-quads/
+  SERD_TRIG         = 4  ///< Terse quads http://www.w3.org/TR/trig/
 } SerdSyntax;
 
 /// Flags indicating inline abbreviation information for a statement
@@ -461,8 +462,8 @@ serd_byte_sink_free(SerdByteSink* SERD_NULLABLE sink);
 /**
    Get a syntax by name.
 
-   Case-insensitive, supports "Turtle", "NTriples", "NQuads", and "TriG".  Zero
-   is returned if the name is not recognized.
+   Case-insensitive, supports "Turtle", "NTriples", "NQuads", and "TriG".
+   `SERD_SYNTAX_EMPTY` is returned if the name is not recognized.
 */
 SERD_PURE_API
 SerdSyntax
@@ -471,8 +472,8 @@ serd_syntax_by_name(const char* SERD_NONNULL name);
 /**
    Guess a syntax from a filename.
 
-   This uses the file extension to guess the syntax of a file.  Zero is
-   returned if the extension is not recognized.
+   This uses the file extension to guess the syntax of a file.
+   `SERD_SYNTAX_EMPTY` is returned if the extension is not recognized.
 */
 SERD_PURE_API
 SerdSyntax
diff --git a/src/serdi.c b/src/serdi.c
index 72b7afe1..e546f463 100644
--- a/src/serdi.c
+++ b/src/serdi.c
@@ -31,6 +31,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 
 #define SERDI_ERROR(msg) fprintf(stderr, "serdi: " msg)
 #define SERDI_ERRORF(fmt, ...) fprintf(stderr, "serdi: " fmt, __VA_ARGS__)
@@ -95,14 +96,15 @@ main(int argc, char** argv)
     return print_usage(argv[0], true);
   }
 
-  SerdSyntax      input_syntax  = (SerdSyntax)0;
-  SerdSyntax      output_syntax = (SerdSyntax)0;
+  SerdSyntax      input_syntax  = SERD_SYNTAX_EMPTY;
+  SerdSyntax      output_syntax = SERD_SYNTAX_EMPTY;
   SerdReaderFlags reader_flags  = 0;
   SerdWriterFlags writer_flags  = 0;
   bool            from_string   = false;
   bool            from_stdin    = false;
   bool            bulk_read     = true;
   bool            bulk_write    = false;
+  bool            osyntax_set   = false;
   bool            quiet         = false;
   size_t          stack_size    = 4194304;
   const char*     add_prefix    = NULL;
@@ -158,11 +160,14 @@ main(int argc, char** argv)
       }
       stack_size = (size_t)size;
     } else if (argv[a][1] == 'o') {
+      osyntax_set = true;
       if (++a == argc) {
         return missing_arg(argv[0], 'o');
       }
 
-      if (!(output_syntax = serd_syntax_by_name(argv[a]))) {
+      if (!strcmp(argv[a], "empty")) {
+        output_syntax = SERD_SYNTAX_EMPTY;
+      } else if (!(output_syntax = serd_syntax_by_name(argv[a]))) {
         return print_usage(argv[0], true);
       }
     } else if (argv[a][1] == 'p') {
@@ -206,7 +211,7 @@ main(int argc, char** argv)
   }
 
   const bool input_has_graphs = serd_syntax_has_graphs(input_syntax);
-  if (!output_syntax) {
+  if (!output_syntax && !osyntax_set) {
     output_syntax = input_has_graphs ? SERD_NQUADS : SERD_NTRIPLES;
   }
 
diff --git a/src/syntax.c b/src/syntax.c
index 61814709..59453bca 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -32,7 +32,7 @@ static const Syntax syntaxes[] = {
   {"ntriples", ".nt", SERD_NTRIPLES},
   {"nquads", ".nq", SERD_NQUADS},
   {"trig", ".trig", SERD_TRIG},
-  {NULL, NULL, (SerdSyntax)0},
+  {NULL, NULL, SERD_SYNTAX_EMPTY},
 };
 
 SerdSyntax
@@ -43,7 +43,7 @@ serd_syntax_by_name(const char* const name)
       return s->syntax;
     }
   }
-  return (SerdSyntax)0;
+  return SERD_SYNTAX_EMPTY;
 }
 
 SerdSyntax
@@ -52,12 +52,12 @@ serd_guess_syntax(const char* const filename)
   const char* ext = strrchr(filename, '.');
   if (ext) {
     for (const Syntax* s = syntaxes; s->name; ++s) {
-      if (!serd_strncasecmp(s->extension, ext, strlen(ext))) {
+      if (s->extension && !serd_strncasecmp(s->extension, ext, strlen(ext))) {
         return s->syntax;
       }
     }
   }
-  return (SerdSyntax)0;
+  return SERD_SYNTAX_EMPTY;
 }
 
 bool
diff --git a/src/writer.c b/src/writer.c
index 71766d9d..afeee33d 100644
--- a/src/writer.c
+++ b/src/writer.c
@@ -843,6 +843,8 @@ write_curie(SerdWriter* const        writer,
     if (is_inline_start(writer, field, flags)) {
       TRY(st, esink(" ;", 2, writer));
     }
+  case SERD_SYNTAX_EMPTY:
+    break;
   }
   return st;
 }
@@ -946,6 +948,10 @@ serd_writer_write_statement(SerdWriter*          writer,
   assert(!((flags & SERD_ANON_S) && (flags & SERD_LIST_S)));
   assert(!((flags & SERD_ANON_O) && (flags & SERD_LIST_O)));
 
+  if (writer->syntax == SERD_SYNTAX_EMPTY) {
+    return SERD_SUCCESS;
+  }
+
   SerdStatus            st        = SERD_SUCCESS;
   const SerdNode* const subject   = serd_statement_subject(statement);
   const SerdNode* const predicate = serd_statement_predicate(statement);
@@ -1093,7 +1099,7 @@ serd_writer_write_statement(SerdWriter*          writer,
 SERD_WARN_UNUSED_RESULT static SerdStatus
 serd_writer_end_anon(SerdWriter* writer, const SerdNode* node)
 {
-  if (writer->syntax == SERD_NTRIPLES || writer->syntax == SERD_NQUADS) {
+  if (writer->syntax != SERD_TURTLE && writer->syntax != SERD_TRIG) {
     return SERD_SUCCESS;
   }
 
diff --git a/test/meson.build b/test/meson.build
index 3926888d..49a3fe0c 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -107,6 +107,14 @@ if get_option('utils')
        should_fail: true,
        suite: ['serdi', 'input'])
 
+  # Output
+
+  test('empty', files('test_empty.py'),
+       args: script_args + [serd_ttl],
+       suite: 'output')
+  
+  # FIXME: Old base URI argument?
+
   # IO errors
 
   test('read_dir', serdi,
diff --git a/test/test_empty.py b/test/test_empty.py
new file mode 100755
index 00000000..a7978e6c
--- /dev/null
+++ b/test/test_empty.py
@@ -0,0 +1,27 @@
+#!/usr/bin/env python3
+
+"""Test writing empty output."""
+
+import argparse
+import sys
+import shlex
+import subprocess
+import tempfile
+
+parser = argparse.ArgumentParser(description=__doc__)
+
+parser.add_argument("--serdi", default="./serdi", help="path to serdi")
+parser.add_argument("--wrapper", default="", help="executable wrapper")
+parser.add_argument("input", help="valid input file")
+
+args = parser.parse_args(sys.argv[1:])
+command = shlex.split(args.wrapper) + [args.serdi, "-o", "empty", args.input]
+
+with tempfile.TemporaryFile() as out:
+
+    proc = subprocess.run(command, check=False, stdout=out)
+
+    out.seek(0, 2)  # Seek to end
+
+    assert proc.returncode == 0
+    assert out.tell() == 0