Implicit block overhaul

[satyr]

The current rule for generating implicit block is too complicated to explain in a few sentences.

For example the following:

switch
case a then if b then c else d

doesn't parse, because the first then generates DEDENT before else.
You pretty much have to read through addImplicitIndentations to grok the rule.

So here's a proposal to make it simpler:

1. Insert an implicit INDENT after an implicit block opener (then -> etc.) if the next token isn't NEWLINE or INDENT.
2. Insert an implicit DEDENT before an implicit block closer (else catch etc.) if the last INDENT is implicit.

E.g.:

-> a
   b
# parses as
->
   a
   b

switch
case a then if b then c else d
# parses as
switch
case a
            if b
                      c
            else
                             d

Pros

Simpler, grokkable rule.

Cons

Incompatibility. Some code currently valid like

for k in o then let
  ...

will be invalid.

[satyr]

To compensate the loss of then if then let etc., we can backport LiveScript's =>:

truthies = for x of xs => if x => x

The pipe operator can be |>.

[satyr]

• 7ebcdbc fixed the OP test case.
• d0a3b1d enabled the Haskell-ish off-side rule via =>.

Making this less attractive, especially with the then-if incompatibility.