#!/bin/bash -e
## Right-fold HOF, with stdio as the second datatype.
##
## Usage:      foldr grep -- x y z
## expands to: grep x | grep y | grep z
##
## Note this is NOT the same as 'x|y|z'.
##
## Example: finding all five-letter words containing all the letters
## from "mail".
##
##     </usr/share/dict/words egrep -x '.{5}' | foldr grep -- m a i l

f=()
while (( $# ))
do  if [[ $1 = -- ]]
    then
        shift
        case $# in
            0)  exit 1;;        # foldr grep [] = \turnstile
            1)  "${f[@]}" "$1";;
            *)  "${f[@]}" "$1" |
                "$0" "${f[@]}" -- "${@:2}";;
        esac
        exit
    else
        f+=("$1")
        shift
    fi
done
exit 2                          # foldr = partial function, i.e. fail