Re: [ferret_users] Eddy tracking algorithm

[Ryo Furue <furue@xxxxxxxxxx>]

Hi,

I agree that there are many problems which Ferret is not a suitable programming language to solve in.  Here I'd just like to comment on the following issue.

> 1. Delayed mode evaluation
>      Ferret evaluates all expressions only when it meets an action command (Ch3 Sec2.  EXPRESSIONS).
>      This means that you cannot use expressions like 
> 
>            let var = var * something_else
>      
>      One may think this is not a serious problem and a new variable name can be used for such steps.
>      But in experience, this is a serious problem.

That is a problem, for example, when you want to loop.  But, lazy evaluation is necessary in Ferret:

  let speed = (u*u + v*v)^0.5 ! big 4D arrays
  fill/k=1/l=1 speed ! I only need a slice

Ferret computes only the necessary part of the big array, which is a blessing.  In an eager-evaluation language like C and Fortran, the first line would consume a lot of time and memory.  You can't do this

  let var = var + something

because the (partial) evaluation of the right-hand side occurs only when its value is required.  In other words, "let" is a "definition", not an "assignment", in Ferret.

In Ferret, you don't usually use loops; you use functions.  In such a case, lazy evaluation saves time and memory because computation occurs only when necessary.  (There are computer languages that behave like that; Haskell is a primary example. You don't use loops in such a language; you use functions.)

> Another aspect is that you cannot use a variable
>      name which is already there in any of the opened datasets.

I think this is a separate issue, and I agree that a fine-grained namespace would be very nice for programming. 

Regards,
Ryo