This is just static types
isEven = (x) ->
x % 2 is 0
f :: (!isOdd) -> !isEven
f = (x) -> ... isEven = (x) -> x % 2 is 0
isOdd = (x) -> x % 2 isnt 0
addEvens :: (!isEven) -> !isOdd
addEvens = (x) -> x + 1
I find it confusing that the different syntax highlighting of the two '!' suggests (wrongly) that they have different meanings. Also, logical negation is something often used with isOdd/isEven functions.
• Indentation shifts mean different things depending on context (could mean we're doing an object literal, could mean we're defining a function, could mean we're entering a loop, etc.)
• Looping through arrays uses "in" while looping through everything else uses "of"
• Array and object literal syntax might create arrays and objects or define variables depending on which side of the equals sign you're reading
Scala can make this really transparent and concise using an abstract class to implement most of the plumbing and implicit type conversion to automatically convert the contract enforcement types to and from the native types.
case class EvenNumber(val x: Int) extends Contract[Int] {
require(x % 2 != 0)
}There are good uses for static type checkers and design by contract, but neither one is a good substitute for the other.
They both work to enforce program invariants and there's a large overlap in the invariants they can express. Usually contracts are checked dynamically (as in contracts.coffee) but this is not a hard requirement (for example .net's code contracts can be statically checked [1]).
To quote Jeremy Ashkenas's JSConf talk on forking CoffeeScript: "when in doubt, cheat." ;-P
But we would still like to express and enforce some invariants about our code. And we'd like to do it in a more elegant way than constantly writing "if(argument === ...) then ... else ..."
Contracts also allow us to check invariants that might not result in a run-time exception but could lead to the code just being subtly wrong.
Indeed it would be. That said, CoffeeScript is a compiler of sorts.
> And we'd like to do it in a more elegant way than constantly writing "if(argument === ...) then ... else ..."
This doesn't really seem to solve that problem. It just throws consistent errors if the types don't match. Your code would likely throw its own error if you didn't check the type manually. In my experience, native js errors are as easy or easier to track down that throwing errors yourself.
> Contracts also allow us to check invariants that might not result in a run-time exception but could lead to the code just being subtly wrong.
Ah, ok. That makes more sense. They're like python decorators, only single purpose.
The fact that we have a compiler doesn't give us all that much. Adding types to a dynamic language (where things can be monkey patched, evaled, and just generally mutated willy-nilly) is a hard problem (not impossible but definitely research [1 and 2 to start, but much more]).
>> And we'd like to do it in a more elegant way than constantly writing "if(argument === ...) then ... else ..."
> This doesn't really seem to solve that problem.
It solves the problem of elegance :)
> It just throws consistent errors if the types don't match. Your code would likely throw its own error if you didn't check the type manually. In my experience, native js errors are as easy or easier to track down that throwing errors yourself.
Key word there is likely. Like I mentioned, there are some errors that don't result in a run-time error and we might like to check for those too. As a silly example, consider a binary search tree [3] that you want to keep balanced. If you get the balance wrong, there's no immediate fault (no TypeError/NPE).
> Ah, ok. That makes more sense. They're like python decorators, only single purpose.
I guess you could say that. If we had an expressive enough decorator-like system for CoffeeScript we could probably implement contracts with it. CS doesn't have one of course so path of least resistance is to add the contract language extension directly.
[1] http://ecee.colorado.edu/~siek/blame-forall-2011.pdf
[2] http://dl.acm.org/citation.cfm?id=1863561
[3] https://github.com/disnet/contracts.coffee/blob/master/test/...