Block lambda is cool, its syntax isn't

Brendan Eich brendan at
Thu Jan 12 21:20:27 PST 2012

We have three coarse-grained alternatives:

1. ? (params) { body }

1a. One problem is what should we use for ?? 'lambda', 'block', etc. are 
not reserved the syntax. Whatever the identifier (including Greek 
lambda: ?), this is an incompatible change.

1b. Another problem: this form looks like a function expression with a 
different introductory keyword, but Tennent's Correspondence Principle 
makes the meaning of body elements including |this| and 
return/break/continue radically different. Different-enough semantics 
should have different syntax.

2. { (params) body }

Putting the parameter list in parentheses is more consistent with 
function syntax, while putting the parameters on the inside stresses the 
block-ness over function-ness. By block-ness I mean (ignoring let, 
const, and function in Harmony) how code and {code} are equivalent.

Of course this cuts against the syntax too: block statements are not 
first-class callables whose code bodies are evaluated only if invoked, 
they are statements evaluated eagerly when reached. This syntax is 
arguably too block-like.

A point that I remember having come up in past TC39 meetings: if we ever 
want structuring forms (object literals or destructuring patterns) that 
use () for grouping property names, this syntax is future hostile.

3. { |params| body }

This is new-to-JS, idiomatic block-like -- but not too 
block-statement-like -- syntax for novel body semantics.

The bars will drive some people to distraction. Others will grok, or 
already do via Ruby. This is not to favor Ruby, just to build on the 
belief that some precedentin language design is better than none.

We could reject (3) in favor of (2) if we had an overriding 
non-aesthetic reason. I don't see one.

Aesthetics vary. The more conventional look of (2) seems like a 
usability hazard to me, which trumps aesthetics. Again I'd rather have 
something more different-looking to signal the novel application of TCP.


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