improve 'convert' example

it can now convert between any two supported formats

sophia/examples/convert.rs

@@ -1,36 +1,110 @@
-//! Convert turtle on stdin into another format
+//! Convert RDF on stdin from one format to another
+use resiter::map::Map;
 use sophia::serializer::TripleSerializer;
-use sophia::triple::stream::TripleSource;
+use sophia::term::{BoxTerm, CopyTerm};
+use sophia::triple::{stream::TripleSource, Triple};
+use std::convert::Infallible;
 use std::io;
+use std::iter::IntoIterator;
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
     let args: Vec<_> = std::env::args().collect();
-    match args.get(1).map(String::as_str).unwrap_or("") {
+    if args.len() != 3 {
+        let message = if args.len() < 3 {
+            "too few arguments"
+        } else {
+            "too many arguments"
+        };
+        exit_printing_usage(&args, message, Status::ArgNumber);
+    }
+    match args[2].as_str() {
         "nt" => {
             let mut ser = sophia::serializer::nt::NtSerializer::new(io::stdout());
-            ser.serialize_triples(get_triples())?;
+            ser.serialize_triples(get_triples(&args))?;
         }
         "turtle" => {
             let mut ser = sophia::serializer::turtle::TurtleSerializer::new(io::stdout());
-            ser.serialize_triples(get_triples())?;
+            ser.serialize_triples(get_triples(&args))?;
         }
         #[cfg(feature = "xml")]
         "xml" => {
             let mut ser = sophia::serializer::xml::RdfXmlSerializer::new(io::stdout());
-            ser.serialize_triples(get_triples())?;
+            ser.serialize_triples(get_triples(&args))?;
+        }
+        #[cfg(not(feature = "xml"))]
+        "xml" => {
+            exit_printing_usage(
+                &args,
+                "RDF/XML not supported (recompile with --feature xml)",
+                Status::UnsupportedXml,
+            );
         }
         _ => {
-            #[cfg(feature = "xml")]
-            eprintln!("usage: {} [nt|turtle|xml]", args[0]);
-            #[cfg(not(feature = "xml"))]
-            eprintln!("usage: {} [nt|turtle]", args[0]);
-            std::process::exit(1);
+            exit_printing_usage(&args, "unknown output format", Status::InputFormat);
         }
     }
     Ok(())
 }
 
-fn get_triples() -> impl TripleSource {
+fn get_triples(args: &[String]) -> MyTripleSource {
     let bufread = io::BufReader::new(io::stdin());
-    sophia::parser::turtle::parse_bufread(bufread)
+    match args[1].as_str() {
+        "nt" => wrap(sophia::parser::nt::parse_bufread(bufread)),
+        "turtle" => wrap(sophia::parser::turtle::parse_bufread(bufread)),
+        #[cfg(feature = "xml")]
+        "xml" => wrap(sophia::parser::xml::parse_bufread(bufread)),
+        #[cfg(not(feature = "xml"))]
+        "xml" => {
+            exit_printing_usage(
+                &args,
+                "RDF/XML not supported (recompile with --feature xml)",
+                Status::UnsupportedXml,
+            );
+        }
+        _ => {
+            exit_printing_usage(&args, "unknown output format", Status::OutputFormat);
+        }
+    }
+}
+
+fn wrap<TS: TripleSource + 'static>(ts: TS) -> MyTripleSource {
+    Box::new(
+        ts.map_triples(|t| {
+            [
+                BoxTerm::copy(t.s()),
+                BoxTerm::copy(t.p()),
+                BoxTerm::copy(t.o()),
+            ]
+        })
+        .into_iter()
+        .map_err(|e| {
+            eprintln!("error while parsing: {}", e);
+            std::process::exit(Status::ParseError as i32)
+        }),
+    )
+}
+
+type MyTripleSource = Box<dyn Iterator<Item = Result<[BoxTerm; 3], Infallible>>>;
+
+fn exit_printing_usage(args: &[String], message: &str, status: Status) -> ! {
+    if message.len() > 0 {
+        eprintln!("{}", message);
+    }
+    eprintln!("usage: {} <format-in> <format-out>", args[0]);
+    eprintln!("  convert RDF on STDIN from one format to another,");
+    eprintln!("  where format can be:");
+    eprintln!("    - nt");
+    eprintln!("    - turtle");
+    #[cfg(feature = "xml")]
+    eprintln!("    - xml");
+    std::process::exit(status as i32)
+}
+
+enum Status {
+    ArgNumber = 1,
+    InputFormat = 2,
+    OutputFormat = 3,
+    ParseError = 4,
+    #[allow(dead_code)]
+    UnsupportedXml = 5,
 }