DAP4 Commentary: Possible Notation for Server Commands

Looking to the future, it is clear that eventually our query language, or more generically our previous discussion of URL Annotations must encompass three classes of computations.
  1. Queries in the DAP2 sense,
  2. Commands to control the client-side processing of requests on the server (i.e. thing like caching),
  3. Server-side processing.
I want to propose a notation for everything in the URL after the "?". I think this notation has ability to represent a wide variety of features without, I hope, being too generic.

The notation is basically nested functions combined with single assignment variables. A semantically nonsensical, but grammatical example would look something like this: "?_x=f(17,g(h(12))),f2(_x,[0:3:10])".

Everything past the "?" is in the form of a comma separated list of nested function invocations. Anything that begins with an underscore is considered a local, temporary, variable, anything that does not look like a function call (i.e. that is not a name followed immediately by a left parenthesis) is assumed to be a string constant. Each function has an arbitrary number of argument expressions separated by commas.

There would be several semantic rules.

  1. A variable may only be assigned to once (single assignment), but may be referenced as many times as desired after that.

  2. All functions have a defined "return type", which looks like a legal DDX minus certain things like groups, enumeration declarations, and dimension declarations. In addition, a function may be defined to have a "void" return type, which means it is executed for its side-effects on the server.

  3. Any expression that is not assigned to a variable and does not have a void return type will have its return value returned to the caller as part of a DATADDX.

Notes

My hypothesis is that this notation should also be able to handle most kinds of server side processing by defining and composing functions.

The standard projection+selection constraints of DAP2 can be represented using a special query() function whose argument is the standard DAP2 constraint, or alternatively, one could define a collection of nested functions to do the same thing, or alternatively, we could split the query part into two pieces separated by a semicolon. The first piece would be a constraint expression and the second piece (after the semicolon) would be in the nest function call form defined above.

An important aspect has to do with the construction of what may be referred to as a DATADDX. It defines the structure of a DDX that is the composition of the return types of the invoked functions that will return a (possibly structured) value. I need to work this out. BUT, in any case, the resulting DATADDX may have only have a loose relation to any DDX representing the raw dataset. This is because server-side computations will not have been represented in the original DDX, but only in the DATADDX.

I also hypothesize that Ferret notations

 http://.../thredds/dodsC/hfrnet/agg/6km_expr_{}{let deq1ubar=u[d=1,l=1:24@ave]}
could be represented in my proposed function notation without having to clutter up the URL format.
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