Expand bash array into curly braces syntax

Question

What would be a nice way to take a bash array variable:

lst=(a b c)

and turn it into a curly brace syntax, such that it gives a statement equivalent to:

$ echo foo{a,b,c}
fooa foob fooc

? Naturally, I'm looking to replace the curly braces with the expansion of lst. In my specific case, it is not possible (or rather, extremely ugly) to use a for loop.

Closest I got so far is:

$ lst=(a b c); echo foo${lst[@]}
fooa b c

Answer 1

Recall that if ary is an array, then the expansion of ${ary[@]/#/foo} is that of ary, with foo prepended to each field. Similarly, ${ary[@]/%/foo} appends foo to each field. Look:

$ lst=( a b c )
$ echo "${lst[@]/#/foo}"
fooa foob fooc
$ echo "${lst[@]/%/foo}"
afoo bfoo cfoo

You don't need eval or printf for that.

Like so, you can safely build arrays:

$ lst=( a 'field with space' b )
$ foolst=( "${lst[@]/#/foo}" )
$ declare -p foolst
declare -a foolst='([0]="fooa" [1]="foofield with space" [2]="foob")'