I was used to write a lot of PHP code, mostly using the symfony framework. When I discovered unit testing with PHP, I was like I had discovered a new way of programming.
Unit testing: your own compiler
It was not required anymore to open a web browser to check if my code was working, I just had to launch a command line, and the red or green lights were telling me if my code was working (or at least, not throwing trivial warnings, notices or exceptions). It was a revolution for me.
It took me a while to realize that for each and every project that I was doing in PHP, half of my tests was doing the job of a compiler (checking the expected output, checking that methods names have been changed everywhere when refactoring, ...). What was the point in re-inventing the wheel?
Type inference: a taste of dynamic language
I was re-inventing the wheel because languages with a compiler were (to me) really verbose for a little added value. Having to tell to Java that "foo" must be treated as a string and 2 as an integer, and 3.2 as a float number was adding a lot of boilerplate to the code:
String test = "foo"; int two = 2; float three = 3.2;
Using PHP it was more easy:
$test = "foo"; $two = 2; $three = 3.2;
But what I loosed in PHP was that substracting a number to a string should not be permitted:
$bar = $test - $two;
But obviously in PHP it just works. I mean, it does not complain:
And it displays ... the number -2 (seriously? ;) ).
So what about something like that (example in Scala):
val test = "foo" val two = 2 val three = 3.2
You don't have to specify the types (like in PHP), but it complains when you try to do so:
val bar = test - two
Here is the error message when you try to compile:
error: value - is not a member of java.lang.String val bar = test - two ^ one error found
You can see that the compiler as inferred the type of test as a String without having to tell it explicitly.
The compiler is your friend
If you are using a language like PHP (or some other dynamic language) and you are writing tons of unit tests to cover your (hypothetical? ;) ) refactoring, specifying types hints in function parameters (typical in PHP those days) you should really ask yourself if you are using the right tool for the right job.
Using a srongly typed language that is using type inference (like Scala or Haskell) could save you plenty of time. You'll have the unique sensation that: "once it compiles, it will most likely work". You should think twice about it.