1. Your first AI function
Eighty customer messages, four teams, and a function whose body is a language model. By the end you will have sorted every message, counted how many it got right, made it better, and know what it cost to the cent.
A small homeware shop gets messages all day: a parcel that never came, a card charged twice, a kettle that won't boil, a password that won't work. Someone reads each one and forwards it to the team that can help: shipping, billing, product or account. Reading eighty messages is a morning. Reading eighty thousand is a job nobody wants.
You are going to write the function that does the reading, and use it like any other Julia function. This is where we are going:
@enum Team shipping billing product account
@ai function team(message::String)::Team
"Which team should answer this customer message?"
end
tickets.team = team.(tickets.message)A column of text goes in; a column of Teams comes out, one model call per row. Everything else in this tutorial is about trusting that column.
You will:
- write an AI function with
@ai, and call it on one message and on a whole column; - see exactly what the model reads, before paying for anything;
- measure how often it is right, and make it better by writing down what you know;
- read the call log to count what it cost.
What you need
- Julia 1.10 or later. FunctAI, and the two packages it uses to talk to models (lmcc and lm15), install from GitHub; add them in this order, since none is in the General registry yet:
using Pkg
Pkg.add(url = "https://github.com/MaximeRivest/lmcc", subdir = "julia")
Pkg.add(url = "https://github.com/lm15-dev/LM15.jl")
Pkg.add(url = "https://github.com/MaximeRivest/functai", subdir = "julia")
Pkg.add(["DataFrames", "CairoMakie"])- A key for a model provider. This series mostly uses OpenAI's. Set it in the shell that starts Julia (
export OPENAI_API_KEY=sk-...), or in~/.julia/config/startup.jl(a file that is never shared or committed):
ENV["OPENAI_API_KEY"] = "sk-..."- Less than a cent of model calls. You will see the exact bill at the end.
This tutorial assumes you know a little DataFrames.jl (making a column, groupby and combine). Nothing else.
Setting up
using FunctAI, DataFrames, CairoMakie, Statistics
log_folder = mktempdir()
FunctAI.configure!(lm = "gpt-6-luna", log_calls = log_folder);FunctAI.configure! sets choices for the whole session (the ! says it changes something, as always in Julia):
lmis the language model that will do the work.gpt-6-lunais OpenAI's smallest current model (September 2026): fast, and cheap enough that a thousand messages cost a few cents.log_callskeeps a record of every call in a folder. We will read it at the end to count what we spent. (mktempdir()makes a fresh, empty folder, so this tutorial only counts its own calls.)
The messages
FunctAI comes with the shop's messages as a dataset, FunctAI.tickets(), a table any Julia table package can read. A person has already decided which team should answer each one: that's the category column. We won't show it to the model; it's the answer key.
tickets = DataFrame(FunctAI.tickets())80×5 DataFrame
Row │ id message channel category order_id
│ Int64 String String String String?
─────┼───────────────────────────────────────────────────────────────────────
1 │ 1 Hi, my order A-1042 still hasn't… email shipping A-1042
2 │ 2 The mug arrived in pieces. chat shipping missing
3 │ 3 I was charged twice for order B-… email billing B-2210
4 │ 4 How do I change the email on my … chat account missing
5 │ 5 The kettle lid doesn't close pro… email product missing
6 │ 6 I'd like my money back for the t… email billing missing
7 │ 7 Tracking for C-3319 hasn't moved… chat shipping C-3319
8 │ 8 I forgot my password and the res… chat account missing
⋮ │ ⋮ ⋮ ⋮ ⋮ ⋮
74 │ 74 Return the headphones and give m… chat billing missing
75 │ 75 Can the dutch oven go on an indu… chat product missing
76 │ 76 I changed my email and now I can… email account missing
77 │ 77 The ceramic bowls came smashed, … email shipping C-3555
78 │ 78 I want to cancel my subscription… email billing missing
79 │ 79 The knife rusted after I left it… chat product missing
80 │ 80 Please send the password reset t… chat account missing
65 rows omittedcombine(groupby(tickets, :category), nrow => :n)4×2 DataFrame
Row │ category n
│ String Int64
─────┼─────────────────
1 │ shipping 22
2 │ billing 22
3 │ account 18
4 │ product 18A function with no body
First the answers: four teams, as an @enum, Julia's type for "one of these names":
@enum Team shipping billing product accountThen the function:
@ai function team(message::String)::Team
"Which team should answer this customer message?"
endAI function team(message::String) -> Team
model: gpt-6-luna
instruction: Which team should answer this customer message?
version: sha256:99ae724adb8e…
see: FunctAI.prompt(team, …) for the exact requestRead it as any Julia function:
team(message::String)takes one argument, aString.::Teamsays what comes back: one of the four teams. The model may only answer one of them, and you get aTeam, not text.- The string in the body says what it does, the way you'd explain the job to a new colleague.
There is no code in the body for you to write. @ai makes a function whose body a language model writes, every time it is called. Printing it shows what you declared and which model will do the work. It really is a function:
team isa FunctiontrueWhat the model reads
A language model reads text and writes text. So what text does team send? FunctAI.prompt shows the exact request, as a conversation, without sending it (and without paying for it):
FunctAI.prompt(team, "My card was charged twice for order B-2210.")model: gpt-6-luna
system
Function: team
Which team should answer this customer message?
Reply in exactly this form:
<result>
one of: shipping, billing, product, account
</result>
user
<message>
My card was charged twice for order B-2210.
</message>The system message is the instruction, written from your function: its name, your sentence, and the form the reply must take. The user message is the input. When the reply comes back, FunctAI reads the text between <result> and </result>, checks it is one of the four teams, and hands you a Team. A reply that doesn't fit is asked again once; if it still doesn't fit, you get an error (or, over a column, a missing), never a made-up value.
One call
team("My card was charged twice for order B-2210.")billing::Team = 1That took a second or two: the question went to OpenAI's servers, the model thought about it, and the answer came back as a Team (the REPL shows an @enum value with its number, = 1; it is the value billing). The very first call of a session also waits while Julia compiles; the calls after it don't.
A whole column
A dot calls any Julia function on every element (uppercase.(names)), and so it does team: eighty messages in, eighty answers back, in order.
tickets.guess = team.(tickets.message)
select(tickets, :category, :guess, :message)80×3 DataFrame
Row │ category guess message
│ String Team String
─────┼───────────────────────────────────────────────────────
1 │ shipping shipping Hi, my order A-1042 still hasn't…
2 │ shipping shipping The mug arrived in pieces.
3 │ billing billing I was charged twice for order B-…
4 │ account account How do I change the email on my …
5 │ product product The kettle lid doesn't close pro…
6 │ billing billing I'd like my money back for the t…
7 │ shipping shipping Tracking for C-3319 hasn't moved…
8 │ account account I forgot my password and the res…
⋮ │ ⋮ ⋮ ⋮
74 │ billing billing Return the headphones and give m…
75 │ product product Can the dutch oven go on an indu…
76 │ account account I changed my email and now I can…
77 │ shipping shipping The ceramic bowls came smashed, …
78 │ billing billing I want to cancel my subscription…
79 │ product product The knife rusted after I left it…
80 │ account account Please send the password reset t…
65 rows omittedThat was eighty model calls. FunctAI sends up to eight at a time (the concurrency setting), so it took seconds, not minutes. The same thing in DataFrames' own words is transform(tickets, :message => ByRow(team) => :guess), and it is just as concurrent. guess is an ordinary column of Teams, so everything you know works on it:
counts = sort(combine(groupby(tickets, :guess), nrow => :n), :n)
fig = Figure(size = (700, 240))
ax = Axis(fig[1, 1], xlabel = "messages", ylabel = "the team the model chose",
yticks = (1:nrow(counts), string.(counts.guess)))
barplot!(ax, counts.n; direction = :x)
fig
Was it right?
We have a person's answer (category, text) next to the model's (guess, a Team), so "how often is it right?" is a proportion:
right = string.(tickets.guess) .== tickets.category
(right = count(right), n = length(right), accuracy = mean(right))(right = 77, n = 80, accuracy = 0.9625)A good score for a function you wrote in three lines. But the interesting rows are the wrong ones:
tickets[.!right, [:category, :guess, :message]]3×3 DataFrame
Row │ category guess message
│ String Team String
─────┼───────────────────────────────────────────────────────
1 │ billing shipping Why was I charged for shipping w…
2 │ billing product Money back please, the knife set…
3 │ billing product The duvet shrank in the wash, I'…Read them next to the shop's house rules (they're in ?FunctAI.tickets). Two rules trip up anyone who hasn't read them:
- anything that arrived broken is shipping, because the carrier pays;
- every request for money back is billing, whatever the reason.
A new colleague would make the same sensible guesses the model made, and they wouldn't be the shop's. The model doesn't know the rules either.
Tell it what you know
The sentence in the body is the function's code. The most direct fix is to write the rules into it:
@ai function team_rules(message::String)::Team
"""
Which team should answer this customer message?
House rules:
- Anything wrong with the delivery itself (late, lost, wrong address, wrong item,
something missing, or broken when it arrived) is shipping: the carrier pays.
- Anything about money (charges, invoices, coupons, cards, and every request for
money back, whatever the reason) is billing.
- Problems that appear while using a product, and questions about products, are product.
- Signing in, passwords, profile details, personal data and emails from the shop are account.
"""
end
tickets.guess_rules = team_rules.(tickets.message)
(without_rules = mean(string.(tickets.guess) .== tickets.category),
with_rules = mean(string.(tickets.guess_rules) .== tickets.category))(without_rules = 0.9625, with_rules = 1.0)The rules came from the shop's policy, not from peeking at the wrong answers. One honest caveat: we measured both versions on the same eighty messages we've been staring at. That flatters any change you make. Tutorial 4 shows how to test a change fairly, and tutorial 3 how sure you can be of a score from eighty rows.
The same function, another model
Nothing in team_rules is specific to OpenAI. configure (no !: it changes nothing) makes a copy with other settings, such as another provider's model. Here is a message that sits exactly where two rules meet, asked of both:
vase = "The vase came in pieces, can I get my money back?"
team_claude = configure(team_rules; lm = "claude-haiku-4-5")
(gpt_6_luna = team_rules(vase), claude_haiku = team_claude(vase))(gpt_6_luna = billing, claude_haiku = shipping)(The second call needs an ANTHROPIC_API_KEY. Skip it if you don't have one: nothing below depends on it.)
Broken on arrival says shipping; a request for money back says billing. The shop's answer is billing, because the money rule says "whatever the reason". Two models reading the same rules can land on different sides, because the description never says which rule wins when both apply. That's the most useful thing to learn here: where two rules meet is exactly where a model hesitates. The fix is more words ("if a message asks for money back, it is billing, even when the item arrived broken"), then checking again, on messages you didn't write the rule from.
What it cost
Every call went into the log folder. calls reads it back, one row per call, as a table:
log = DataFrame(calls(folder = log_folder))
select(log, :name, :model, :seconds, :input_tokens, :output_tokens, :total_tokens)163×6 DataFrame
Row │ name model seconds input_tokens output_tokens total_tokens
│ String String Float64 Int64 Int64 Int64
─────┼────────────────────────────────────────────────────────────────────────────────────
1 │ team gpt-6-luna 17.8548 63 26 89
2 │ team gpt-6-luna 1.28176 68 25 93
3 │ team gpt-6-luna 1.30768 57 30 87
4 │ team gpt-6-luna 1.28972 66 26 92
5 │ team gpt-6-luna 1.73444 61 25 86
6 │ team gpt-6-luna 1.49509 64 27 91
7 │ team gpt-6-luna 1.33645 64 42 106
8 │ team gpt-6-luna 1.51314 62 27 89
⋮ │ ⋮ ⋮ ⋮ ⋮ ⋮ ⋮
157 │ team_rules gpt-6-luna 0.79266 168 25 193
158 │ team_rules gpt-6-luna 1.15319 171 27 198
159 │ team_rules gpt-6-luna 0.952375 167 40 207
160 │ team_rules gpt-6-luna 1.19611 166 41 207
161 │ team_rules gpt-6-luna 1.01361 170 29 199
162 │ team_rules gpt-6-luna 1.33275 168 65 233
163 │ team_rules claude-haiku-4-5 1.57499 184 9 193
148 rows omittedProviders charge by the token, a piece of a word (about three quarters of an English word on average), with one price for what you send and a higher one for what the model writes. What it writes includes its hidden reasoning: recent models think before they answer, and you pay for the thinking. That's why we count total_tokens - input_tokens as the output.
Prices change, so write them down with the date you read them:
prices = DataFrame(model = ["gpt-6-luna", "claude-haiku-4-5"], # dollars per million tokens, 2026-09-27
input = [0.10, 1.00],
output = [0.50, 5.00])
bill = leftjoin(log, prices, on = :model)
(calls = nrow(bill),
dollars = sum(bill.input_tokens .* bill.input .+ (bill.total_tokens .- bill.input_tokens) .* bill.output) / 1e6)(calls = 163, dollars = 0.004932199999999999)Keep that in mind when someone says language models are expensive. For sorting short messages, the small ones cost about as much as the electricity to read this page.
Your turn
- Write
urgent, a function that answerstrueorfalse(::Bool): does this message need an answer today? Run it ontickets.messageand count thetrues bycategorywithgroupbyandcombine. Which team gets the most urgent messages? - Look at
FunctAI.prompt(team_rules, "hi"). Where did your house rules go? - Give
team_rulesa message you write yourself that sits between two rules. What does it answer? Would a new colleague agree?
What you learned
@ai function name(input::Type)::Answer "what it does" endwrites a function whose body is a language model. It is a JuliaFunction: call it, broadcast it with a dot, pass it toByRow.- The answer comes back as the type you declared. An
@enummeans the model can only give one of your answers. FunctAI.promptshows exactly what the model will read.- "Is it right?" is a proportion when you have the right answers in a column.
- The description is your function's code. Writing down what you know (the house rules) is the most direct way to make it better.
callsreads the log: calls, time and tokens, and so dollars.
Next: 2. Answers you can compute with turns free-text field notes into a table of numbers, categories and records you can plot.