readline-nonl: Read and print an unterminated line from stdin

Copyright (c) 2018 Bart Massey

This repository contains implementations in more than 11 languages of a program that reads and prints a line from standard input, together with a test script which runs each of these implementations against a test file.

Motivation

One of the things you might do in a programming language is to read a line from standard input. By convention, lines are terminated with a newline character. It is sometimes the case that the last or only line in a file might be unterminated, either accidentally or by design.

If a file with an unterminated line is fed to a program as standard input, the programming language routine for reading a line has to decide what to do with this odd line. Obvious possibilities: pretend the line has a newline at the end, throw some kind of exception. Programming languages pretty uniformly do the former. Go's line reader is peculiar in that it returns both the line contents and an error indication.

A less obvious possibility for reading an unterminated line from standard input was chosen by Google in their d8 Javascript implementation, which silently returns undefined in this case. In Javascript printing undefined is of course a legitimate activity, so it is turned into the string undefined and printed on standard output. Mozilla's smjs Javascript implementation does not exhibit this behavior, returning the line as expected.

Demonstration

The file nonl.txt contains the string "hello" without a terminating newline. This file is used by default in the test script. Running sh test.sh on my system yields this:

./r-c: hello
./r-cc: hello
java r_java: hello
./r-rs: hello
runghc r.hs: hello
perl r.pl: hello
sh r.sh: hello
python3 r3.py: hello
python2 r2.py: hello
smjs r.js: hello
nickle r.5c: hello
go run r.go: hello
read error: EOF
exit status 1
d8 r.js: undefined

Note that the Go program writes the error and exit status indication to standard error, which was captured here.

The file nl.txt contains the string "hello" with a terminating newline. Running sh test.sh nl.txt on my system yields this:

./r-c: hello

./r-cc: hello
java r_java: hello
./r-rs: hello

runghc r.hs: hello
perl r.pl: hello

sh r.sh: hello
python3 r3.py: hello

python2 r2.py: hello

smjs r.js: hello
nickle r.5c: hello
go run r.go: hello

d8 r.js: hello

It is easy to see from this output which languages strip the terminating newline on read and which do not.

Comparison of Languages

While I didn't intend this project as a comparative study of the ergonomics of reading a line in various languages, there is some stuff here that I found interesting.

• The documentation of how to read a line from standard input in various languages ranges from modest to nonexistent. I am familiar with all of these languages (a big part of why I chose them), so I was surprised by how much time I spent on Stack Overflow reminding myself how to perform this simple task.

• C infamously provides no way for its line reader to allocate a string sufficiently long to read the line. I used the usual "pick a size and die if it fails" strategy. Even after 35 years of C programming, I forgot to check the error return from fgets() and had to go back and add it.

• I chose to include Nickle because I am a co-creator of the language (although almost all of the actual implementation is by co-author Keith Packard). I am relieved, though not surprised, by our language's behavior in this common case.

• The command-line Javascript implementation uses primitives provided by both d8 and smjs, but which as far as I know are not part of the Javascript specification and are not (for obvious reasons) available in a web browser implementation of the language. The rhino command-line Javascript implementation throws an error that I don't understand on this program (with a tremendously huge stack trace) even for the newline-terminated file, so I chose not to include it.

• Languages with way more ceremony than I find conscionable for this simple task: Rust, Go, Java. I am working on an RFC for Rust to improve the situation there. There is probably no reasonable plan for Java at this point: I don't know why System.in isn't a BufferedReader to begin with.

• Java's habit of building the class name into the class file is annoying. One would think that one could call the class r, compile with javac r.java, and just rename r.class to r-java.class. Nope: the bytecode interpreter refuses to run it.

Build and Run

You will need installations of all of the above-indicated language implementations on your machine. They are all available as Debian Linux packages with quite guessable names. It is presumably clear how to run test.sh from the comments in the previous section.

Future Work

• I guess it would be worthwhile to add more languages? I ran out of steam at 12.

• I probably should file a bug against d8. Just saying.

License

This program is licensed under the "MIT License". Please see the file LICENSE in the source distribution of this software for license terms.