> On Dec 30, 2017, at 1:23 PM, Joel Dueck <dueckofe...@gmail.com> wrote:
>
> I would have expected define-meta to take the result of the expression rather
> than quoting it. Is there a way to define a meta in terms of an expression
> result?
>
> I don’t have a use for this right now, just curious.
No, `define-meta` can only take a literal value (not an evaluated expression)
for three reasons:
1) For speed, `define-meta` plucks out all the values without evaluating
anything.
2) Evaluating things on the right-hand side of `define-meta` would lead to
sticky questions about what evaluation environment is should be used
(`racket/base`? `pollen/pre`? Can you bring in more imports?)
3) Literal values can be cached to disk (for even more speed).
That said, you aren't limited to symbols, numbers, and strings — you can put
anything on the right side of `define-meta` that Racket understands as a
literal value:
◊(define-meta hash-val #hash((a . 1) (c . 3) (b . 2)))
◊(define-meta regex-val #px"^\\d+$")
Still, if you absolutely positively need to export an expression that's
evaluated at runtime, you can always just use `define` rather than
`define-meta`. In this case, `title` will be automatically exported as "Hello
There":
#lang pollen
◊(define title (string-titlecase "hello there"))
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