Collapsing Functions

A collapsing function is a function whose behavior changes depending upon the circumstances under which it's run. Function collapsing is useful when you find yourself repeatedly checking a variable whose value never changes.

Function collapsing requires some static feature in the environment. A common example is a script that gives the user the option of having "verbose" output.

#!/bin/bash

[[ $1 = -v || $1 = --verbose ]] && verbose=1

chatter() {
  if [[ $verbose ]]; then
    chatter() {
      echo "$@"
    }
    chatter "$@"
  else
    chatter() {
      :
    }
  fi
}

echo "Waiting for 10 seconds."
for i in {1..10}; do
  chatter "$i"
  sleep 1
done

The first time you run chatter(), the function redefines itself based on the value of verbose. Thereafter, chatter doesn't check $verbose, it simply is. Further calls to the function reflect its collapsed nature. If verbose is unset, chatter will echo nothing, with no extra effort from the developer.

FIXME Add more examples!

# Somewhat more portable find -executable
# FIXME/UNTESTED (I don't have access to all of the different versions of find.)
# Usage: find PATH ARGS -- use find like normal, except use -executable instead of
# various versions of -perm /+ blah blah and hacks
find() {
  hash find || { echo 'find not found!'; exit 1; }
  # We can be pretty sure "$0" should be executable.
  if [[ $(command find "$0" -executable 2> /dev/null) ]]; then
    unset -f find # We can just use the command find
  elif [[ $(command find "$0" -perm /u+x 2> /dev/null) ]]; then
    find() {
      typeset arg args
      for arg do
        [[ $arg = -executable ]] && args+=(-perm /u+x) || args+=("$arg")
      done
      command find "${args[@]}"
    }
  elif [[ $(command find "$0" -perm +u+x 2> /dev/null) ]]; then
    find() {
      typeset arg args
      for arg do
        [[ $arg = -executable ]] && args+=(-perm +u+x) || args+=("$arg")
      done
      command find "${args[@]}"
    }
  else # Last resort
    find() {
      typeset arg args
      for arg do
        [[ $arg = -executable ]] && args+=(-exec test -x {} \; -print) || args+=("$arg")
      done
      command find "${args[@]}"
    }
  fi
  find "$@"
}

#!/bin/bash
# Using collapsing functions to turn debug messages on/off

[ "--debug" = "$1" ] && dbg=echo || dbg=:


# From now on if you use $dbg instead of echo, you can select if messages will be shown

$dbg "This message will only be displayed if --debug is specified at the command line

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