How to implement a shim?

Community Support
1

Hello,

 

In general, how does one add to an already existing function’s definition? 

For example, suppose we define the identity function:

f:{x}

And that we wanted f to output one more than it currently does. 

f:{f+1}

The above wouldn’t work; this produces a stack error. 

'stack [1799f:{f+1}

I’m not sure how to redefine a function while referencing its current version and not causing such an error.

 

I acknowledge for this simple example, one could just redefine f:{x+1}. But the idea is to build on the existing f - not to destroy f and recreate it from scratch. That would be a redefinition, not a shim. If anything else may be unclear about my question, please let me know. 

 

Any help or advice would be greatly appreciated.

 

Kind regards,

Michael

2

Save the original definition to another variable and call that at the end of the redefinition as follows:

 

q)f 5 5 q)f_orig:f q)f:{f_orig x+1} q)f 5 6

 

Or alternatively you could use a projection:

 

q)f:{x} q)f 5 5 q)f:{x y+1}[f;] q)f 5 6

 

Note - these methods only allow additional handling to the input to the original function.

3

 

Thank you David!

Best regards,

Michael

4

Just for fun two more concepts to introduce:

  • Composition using @ allows you to chain a sequence of functions
  • value can be passed a symbol variable name and will return it’s contents

 

q)f:{x} q)f:value[`f] +[1] @ q)f {x}@[+[1]] q)f 1 2