Anonymous function works as callback but defined function does not - coffeescript

Question

I have a module that looks like this:
module.exports = AtomMarkdownLabels =
# other stuff here
file_changed_added: (file_path) =>
fs.readFile file_path, 'utf-8', #process_yaml
console.log 'file changed'
process_yaml: (err, data) =>
console.log "process_yaml is called"
I know file_changed_added is being called from some other function and I'm seeing the "file changed" output in the console, but process_yaml isn't if I change file_changed_added to
file_changed_added: (file_path) =>
fs.readFile file_path, 'utf-8', (err, data) =>
console.log "test"
console.log 'file changed'
I see both "test" and "file changed" being called properly. What could be going on?

Answer 1

=> has two slightly different purposes:
When defining a named function (f = => ...) or anonymous function f(x, => ...)), => simply ensures that # inside the function is that same as # in the surrounding context.
When defining a method in a class:
class C
m: => ...
=> ensures that # inside m will be the instance of C.
Both uses are creating a bound function but they're binding to different things.
You're using this structure:
obj =
func: =>
# ...
That's equivalent to this:
f = =>
# ...
obj =
func: f
because you're using a plain old object rather than a class. So what is # outside your AtomMarkdownLabels definition? # won't be anything useful and in particular, it won't be your AtomMarkdownLabels object and it won't have a process_yaml property so #process_yaml inside file_changed_added is going to be undefined or an error.
I'm not sure what specifically Atom wants you to return but a class should work, something like this:
# Use a class so that => does what you're expecting it to do
class AtomMarkdownLabels
# other stuff here
file_changed_added: (file_path) =>
fs.readFile file_path, 'utf-8', #process_yaml
console.log 'file changed'
# you may or may not need => here
process_yaml: (err, data) =>
console.log "process_yaml is called"
# Export an instance of your class
module.exports = new AtomMarkdownLabels
If you want to or must use a plain object then you could bypass # completely and do it like this:
# Just plain old functions so define them that way
process_yaml = (err, data) ->
console.log "process_yaml is called"
file_changed_added = (file_path) ->
fs.readFile file_path, 'utf-8', process_yaml
console.log 'file changed'
module.exports = AtomMarkdownLabels =
# other stuff here
file_changed_added: file_changed_added
or like this:
# Explicitly name your object rather than using #
module.exports = AtomMarkdownLabels =
# other stuff here
file_changed_added: (file_path) ->
fs.readFile file_path, 'utf-8', AtomMarkdownLabels.process_yaml
console.log 'file changed'
process_yaml: (err, data) ->
console.log "process_yaml is called"
This should solve your other CoffeeScript problem from a few days ago too.