This is an erlang version of https://gist.github.com/gigamonkey/6249d85021bc8bf54eb4 (with a minor change to 'combos'/'combineAdjacent' for easier reading)

hundreds.erl

#!/usr/bin/env escript
% Problem statement:
% Write a program that outputs all possibilities to put + or - or nothing between the
% numbers 1, 2, ..., 9 (in this order) such that the result is always 100.
% For example: 1 + 2 + 34 – 5 + 67 – 8 + 9 = 100.

% Generate all combos of digits 1-9 with +, -, or nothing in between.
combos([N])    -> [[N]];
combos([N|Ns]) -> [[N, X] ++ Rest || X <- [plus, minus, empty], Rest <- combos(Ns)].

% Combine adjacent digits into a single number.
combineAdjacent([N, empty, M | Rest]) -> combineAdjacent([N * 10 + M | Rest]);
combineAdjacent([N, Op | Rest])       -> [N, Op | combineAdjacent(Rest)];
combineAdjacent(X) -> X.

% Now eval the resulting thing as a left-associative expression.
eval([N, plus, M | Rest])  -> eval([N + M | Rest]);
eval([N, minus, M | Rest]) -> eval([N - M | Rest]);
eval([N]) -> N.

results(Matches) ->
    lists:foldl(fun(X, Result) ->
        Result ++ [lists:map(fun(plus)  -> " + ";
                                (minus) -> " - ";
                                (N) -> integer_to_list(N)
                             end, X)]
    end, [], Matches).

main([]) ->
    Matches100 = lists:filter(fun(Combined) -> eval(Combined) == 100 end, [combineAdjacent(X) || X <- combos(lists:seq(1, 9))]),
    io:format("~s~n", [string:join(results(Matches100), "\n")]).