Daniel Bünzli wrote:
> 
> Le 25 oct. 08 à 14:43, Martin Jambon a écrit :
> 
>> Now I generally tend to use this:
>>
>> let x =
>>  List.map (
>>    fun z ->
>>      very_blabla
>>      ...
>>  ) my_list
>> in
> 
> I think the best solution is to name your anonymous function, as the
> guidelines suggest [1].

It says: "Justification: Much clearer, in particular if the name given
to the function is meaningful."

I think that's true for typical functional code which follows some clear
logic or model.

In many cases it's not possible to give a meaningful name to such a
function and defining it out of the current block can make things
needlessly hard to follow.

I can think of 4 cases:

1. anonymous function that fits on one line
2. anonymous function that doesn't fit on one line (my original example)
3. named function defined locally using let-in
4. named function defined globally using let

I don't use (3) very much since:
* it still causes the outermost function definition to be very long and
hard to follow like (2),
* and it's okay to define a function globally (4) because there is no
serious risk of global namespace pollution, thanks to the module system.

I think (3) is most useful for defining named functions that depend on a
lot of locally-defined values. This creates a closure, which is often
acceptable performance-wise, instead of having to make each parameter of
the function explicit.

In performance-critical code or maybe imperative code in general, it
feels good to control when closures are created. In such cases, avoiding
local functions helps.


Martin

-- 
http://mjambon.com/