Warning for common CoffeeScript pitfalls

[davidbau]

The big advantage of CoffeeScript - optional parentheses for function invocation - is also a big disadvantage when it comes to allowing ambiguous syntax that does not run the way it reads on the screen.

We should improve the ACE editor coffeescript mode to add warnings in the following situations:

• Function invocation with an argument that contains an expression that contains a non-parenthesesed operator with lower precedence than "a - b". For example "touches a and touches b" which is interpreted as "touches(a and touches(b))" should generate a warning, requiring parentheses.
• Function invocation with an argument with a leading unary minus or unary plus. "a +b" is interpreted as coffeescript as "a(+b)" - an extra space is required for "a + b". This is the worst thing in CoffeeScript, and should generate a warning when it is being interpreted as unary minus or plus.
• Nested invocation of functions with more than one argument. "dot random color, random [1..100]" is interpreted as "dot(random(color, random([1..100])))". This ambiguity is hard to read and we should require our users to use some parentheses around the inner calls.

[davidbau]

See the discussion at https://groups.google.com/forum/#!topic/coffeescript/yUPsELGPCQo - there are several other syntax traps noted there.

Also, they point out CoffeeLint: https://github.com/clutchski/coffeelint - a good approach to investigate would be to integrate CoffeeLint into the ACE coffeescript mode (to provide warnings) and then add what we need to CoffeeLint.