@get! macro

get.jl

is_expr(ex, head::Symbol) = (isa(ex, Expr) && (ex.head == head))
is_expr(ex, head::Symbol, n::Int) = is_expr(ex, head) && length(ex.args) == n

# simple syntax version
macro get!(d, k, default)
    quote
        d, k = $(esc(d)), $(esc(k))
        has(d, k) ? d[k] : (d[k] = $(esc(default)))
    end
end

# fancy syntax version, probably puts too much emphasis on assignment
macro get2!(ex)
    if !is_expr(ex, :(=), 2);  error("@setdefault: not an assignment"); end
    lhs, default = ex.args
    if !is_expr(lhs, :ref, 2); error("@setdefault: single index expected"); end
    d, k = lhs.args
    quote
        d, k = $(esc(d)), $(esc(k))
        has(d, k) ? d[k] : (d[k] = $(esc(default)))
    end
end

test.jl

d = {19=>2}
@show d
@show @get!(d, 8, 19)
@show @get!(d, 19, 7)
@show d

println()

d2 = {19=>2}
@show d2
@show (@get2! d2[8] = 19)
@show (@get2! d2[19] = 7)
@show d2

Output from test.jl:

d   = {19=>2}
@get! d 8 19    = 19
@get! d 19 7    = 2
d   = {8=>19,19=>2}

d2  = {19=>2}
@get2! d2[8] = 19    = 19
@get2! d2[19] = 7    = 2
d2  = {8=>19,19=>2}