Pipe no longer clobbers _, fixes #181

[qqueue]

...Is the end goal here, but I have questions on the approach, if you're willing to help.

The relevant code appears to be:

grammar.co:

    o 'Expression |> Expression' -> Block $1 .pipe $3

ast.co:

  pipe: -> @lines.push Assign(Var \_; @lines.pop!), it; this

I also found scope#temporary for getting a new name, but I'm unsure how to change the _ variable within the right-hand side of the pipe to the temporary binding. Is the implementation of pipe as just the Node#pipe method sufficient to do this, or would it be better to make Pipe extends Node to hold the logic for this?

[qqueue]

Went ahead and implemented a fix in 01bc54c, using scope#temporary \_, which usually generates _$ as the compiled pipee. All tests passing.

Concerns:

• To allow the use of _ in coco while compiling to the pipee, I added a check in Var#compile, which may not be the best way to do this.
• Is _$ obscure enough to not clobber variables? I can kind of see somebody else legitimately using _$, given they are the only two special characters you can use in identifiers. "underscore-money.js", perhaps.

[satyr]

Viability aside, you're right that your proposal needs that kind of implementation.

I can kind of see somebody else legitimately using _$

Haven't seen that myself. Use of trailing $ in variable names is pretty rare, supporting #136.