well-typed vignettes

I present to you some well-typed programs along with associated run-time and compile-time errors for pondering the meaning of well-typed. The context was me trying to understand the meaning of the quote “Well-typed programs cannot go wrong”. The exercise was illuminating for me but when I started writing explanations they got a little too philosophical so I’m just going to present the code and the errors.


puts File.read(ARGV[0])


If the file does not exist then there is nothing to do. All programming languages no matter how good their type system can not reason about what is external to them and so the sane thing to do is throw an error and halt or let the programmer do something with that error. In Ruby’s case it does the right thing and throws an exception

main.rb:1:in 'read': No such file or directory @ rb_sysopen - asdf (Errno::ENOENT)
	from main.rb:1:in '<main>'


Contrary to popular belief Ruby actually has static types it’s just that nothing is done with them until run-time. When an argument of the wrong type is passed to a method we get a type error. The difference is that in a more statically typed language the error would be during compile-time instead of run-time

def to_stdout(filename)
  puts File.read(filename)
main.rb:2:in 'read': no implicit conversion of Fixnum into String (TypeError)
	from main.rb:2:in 'to_stdout'
	from main.rb:4:in '<main>'


import System.IO
import System.Environment

dumpContents :: FilePath -> IO ()
dumpContents fileName = openFile fileName ReadMode >>= 
  hGetContents >>= putStrLn

main :: IO ()
main = getArgs >>= \args -> dumpContents (args !! 0)

Prelude.(!!): index too large

Just as in the Ruby case Haskell will throw a run-time exception when assumption about the external world fail. If I don’t pass enough argument then we get a run-time error

main: Prelude.(!!): index too large

openFile: does not exist (No such file or directory)

Similarly if I pass an argument that corresponds to a file that does not exist we again get a run-time error

main: asdf: openFile: does not exist (No such file or directory)

Notice that up to this point both Haskell and Ruby have failed in exactly the same way with run-time errors.

Couldn’t match type ‘[Char]’ with ‘Char’

So this is the one case where Haskell does one better than Ruby. When I pass the wrong argument to dumpContents instead of getting a run-time error the compiler tells me that something is not lining up properly

[1 of 1] Compiling Main             ( main.hs, main.o )

    Couldn't match type ‘[Char]’ with ‘Char’
    Expected type: FilePath
      Actual type: [String]
    In the first argument of ‘dumpContents’, namely ‘(args)’
    In the expression: dumpContents (args)


All the same things happen with TypeScript except we get to choose how strongly we want to enforce certain constraints. These days TypeScript is my go-to choice for all things related to Javascript.

/// <reference path="/home/david/Downloads/node.d.ts"/>
var fs = require('fs');
fs.readFile(process.argv[2], (error : string, data : Buffer) => {

TypeError: path must be a string

TypeError: path must be a string
    at TypeError (native)
    at Object.fs.open (fs.js:491:11)
    at Object.fs.readFile (fs.js:262:6)
    at Object.<anonymous> (/home/david/test/main.js:3:4)
    at Module._compile (module.js:460:26)
    at Object.Module._extensions..js (module.js:478:10)
    at Module.load (module.js:355:32)
    at Function.Module._load (module.js:310:12)
    at Function.Module.runMain (module.js:501:10)
    at startup (node.js:129:16)

TypeError: Can not read property ‘toString’ of undefined

TypeError: Cannot read property 'toString' of undefined
    at /home/david/test/main.js:4:21
    at fs.js:263:20
    at FSReqWrap.oncomplete (fs.js:95:15)