On 10/04/2013 03:26 PM, Sergey Mironov wrote:
Rather, in Ur/Web, no first-class
functions are allowed to exist in server-side code at runtime. The compiler
can't find a way to optimize your code to hide the first-class functions of
type [css_style -> css_style], since there is currently no optimization to
simplify calls to recursive functions (e.g., [foldr]) statically.
Thanks, that is clear. Is that 'mkcss' approach the only way to
synthesize css styles? I'm going to store heights/widths of images in
the database, that is why I need the way to tweak some styles
dynamically. I already have the code which does it using javascript
FFI but that method is not what I'd like to see in the end.
The basic approach you used in your code seems reasonable. I just
suggest making it less higher-order, as I suggested in the quoted text
below.
Two effective refactorings that come to mind are:
1. Fold over a record instead of a list.
2. Fold over a list of "(n, v) pairs" instead of a list of functions,
calling [mkcss] within the first argument to [List.foldr].
Could you please recommend some reading covering the translation of
functional languages into imperative ones? My knowledge on this topic
is too small, I'd like to learn how we deal with first-class functions
when building C code.
I wouldn't want to claim that much about the Ur/Web compilation strategy
is particularly standard, and I really do want to rework it considerably
at some point, to improve predictability.
Fundamentally, the compiler needs to eliminate all first-class functions
at compile time, for server-side code. You can go a long way toward
understanding how it does that just by imagining that it applies
standard algebraic laws, like this one:
(fn x => e1) e2 = e1 with e2 substituted for x
A similar rule is used to inline the definitions of sufficiently short,
non-recursive functions. Also, the compiler will _specialize_ functions
like [List.foldr] to particular arguments of function type. I think
these ingredients are sufficient to explain why my suggestion "2." above
would help.
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