Problem Statement

Find solutions to the   sum to one hundred   puzzle.

Add (insert) the mathematical operators     +   or   -     (plus or minus)   before any of the digits in the decimal numeric string   123456789   such that the resulting mathematical expression adds up to a particular sum   (in this iconic case,   100).

Example:
Show all output here.

‡   (where   infinity   would be a relatively small   123,456,789)

An example of a sum that can't be expressed   (within the rules of this task)   is:   5074 (which,   of course,   isn't the lowest positive sum that can't be expressed).

Let's start with the solution:

main =>
    L = "23456789",
    gen(L,Str),
    Exp = parse_term(['1'|Str]),
    Exp =:= 100,
    println(['1'|Str]).

gen(L@[_],Str) => Str = L.
gen([D|Ds],Str) ?=> Str = [D|StrR], gen(Ds,StrR).     % no operator
gen([D|Ds],Str) ?=> Str = ['+',D|StrR], gen(Ds,StrR). % insert +
gen([D|Ds],Str) => Str = ['-',D|StrR], gen(Ds,StrR).  % insert -