On Fri, Oct 23, 2009 at 3:16 PM, Howard Lewis Ship <hls...@gmail.com> wrote:
>
> I like to try and keep my level of nesting under control, and this
> often involves hiding or re-structuring the let macro. The for macro
> can implicitly assemble a let macro for you, but with a limitation
> that the :let clause can't be first:
>
> 1:5 user=> (for [:let [z [:foo :bar]] x z] (name x))
> java.lang.IllegalStateException: Can't pop empty vector (repl-1:5)
> 1:6 user=> (for [x [:foo :bar] :let [z (name x)]] z)
> ("foo" "bar")
> 1:7 user=>
>
> Is this limitation intentional? Could the error message be improved?
It's not an intentional limitation. One hint of this is that
:let, :while, and :when all work perfectly fine at the beginning
of a doseq.
Improving the error message would be easy. On the other hand,
a patch to support them at the beginning of a 'for' should be
possible. I think it might be worthwhile, particulary for macros
that generate 'for' forms. The machinery inside a 'for'
expansion is hard to do yourself.
Rich, would you consider a patch to support this?
--Chouser
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