Binds

Patrick Hochstenbach edited this page Apr 16, 2016 · 23 revisions

Binds change the execution context of a Fix script. In normal operation, all Fix functions are executed from the first to the last. For example given the YAML input:

---
colors:
  - red
  - green
  - blue

every Fix functions will be executed one by one on all the colors:

upcase(colors.*)
append(colors.*," is a nice color")
copy_field(colors.*,result.$append)

The first Fix upcase will uppercase all the colors, the second append will add " is a nice color" to all the colors, the last copy_field will copy all the colors to a new field.

But what should you do when you want the three Fix functions to operate on each color separately? First upcase on the first color, append on the first color, copy_field on the first color, then again upcase on the second color, append on the second color, etc.

For this type of operation a Bind is needed using the do notation:

do list(path:colors.*, var:c)
  upcase(c)
  append(c," is a nice color")
  copy_field(c,result.$append)
end

In the example above the list Bind was introduced. The context of the execution of the Bind body is changed. Instead of operating on one Catmandu item as a whole, the Fix functions are executed for each element in the list.

Each Bind changes the execution context in some way. For instance Fix functions could execute queries into database, or fetch data from the internet. These operations can fail when the database is down, or the website couldn't be reached. What should happen in that case in a Fix script? Should the execution be stopped? Or, should there errors be ignored.

my_fix1()
my_fix2()
download_from_internet() # <--- this one failes
process_results()

What should happen in the example above? Should the results be processed when the download_from_internet fails? Using the maybe Bind one can skip Fix functions that fail:

do maybe()
  my_fix1()
  my_fix2()
  download_from_internet() 
  process_results() # <--- this is skipped when download_from_internet fails
end

Binds are also used when creating Fix executables. That are Fix scripts that can be run directly from the command line. In the example below we'll write a Fix script that downloads data from an OAI-PMH repository and prints all the record identifiers:

#!/usr/bin/env catmandu run
do importer(OAI,url: "http://lib.ugent.be/oai") 
  retain(_id)
  add_to_exporter(.,YAML)
end

If this script is stored on a file system as myscript.fix and made executable:

$ chmod 755 myscript.fix

then you can run this script as any other Unix command:

$ ./myscript.fix