Conditions and repetition

Texl has no special tags for conditions or loops. A tag is an expression, so you use Swan's conditional operators and mapping to decide what to write.

Optional text

condition ? text writes text when the condition is true, and nothing otherwise. condition ? text ; other writes other when it is false:

<% stock = 0, vip = TRUE %>Status: <% stock > 0 ? "in stock" ; "sold out" %><% vip ? " (priority shipping)" %>

Status: sold out (priority shipping)

Chain conditions for more cases:

<% size = n -> n < 10 ? "small" ; n < 100 ? "medium" ; "large" %><% size 5 %>, <% size 50 %>, <% size 500 %>

small, medium, large

Optional paragraphs

A text literal can span several lines, so a condition can produce a whole paragraph:

<% overdue = TRUE %>Thank you for your order.
<% overdue ? "
Your payment is overdue. Please pay within
seven days to avoid a late fee.
" %>
Best regards

Thank you for your order.

Your payment is overdue. Please pay within
seven days to avoid a late fee.

Best regards

Remember that Swan text cannot contain " and that %> ends the tag. For longer optional parts, or parts with tags of their own, put them in a separate document and import it conditionally.

Repeating text for each item

To write something for each item of a list:

1. turn the list into a tuple with list.enum;
2. map a function that makes the text for one item over the tuple, with =>;
3. join the results with text.join.

<%
fruits = ["apples", "pears", "plums"],
nl = "
"
%>Shopping list:
<% text.join nl (list.enum fruits => (f -> "- " + f)) %>

Shopping list:
- apples
- pears
- plums

Binding the line break to a name, like nl here, keeps the tags readable. Without text.join, the items would be written separated by commas, the way tuples are rendered.

The items are often namespaces, and the function picks their members. Numbers must be converted with text.from before they are joined to text:

<%
nl = "
",
orders = [{id: "A-1", total: 25}, {id: "B-7", total: 140}],
row = o -> "| " + o.id + " | " + text.from o.total + " |"
%>| Order | Total |
|-------|-------|
<% text.join nl (list.enum orders => row) %>

| Order | Total |
|-------|-------|
| A-1 | 25 |
| B-7 | 140 |

Empty lists

list.enum [] is Nothing, so an empty list writes nothing. To write a message instead, test the size:

<% items = [] %><% list.size items == 0 ? "No items." ; text.join ", " (list.enum items) %>

No items.

Filtering

A function that returns Nothing for some items drops them, because Nothing disappears from tuples:

<%
orders = [{id: "A-1", total: 25}, {id: "B-7", total: 140}],
big = o -> o.total > 100 ? o.id
%>Large orders: <% list.enum orders => big %>

Large orders: B-7

Totals

Swan has no loop and no built-in sum, but a short recursive function does the job:

<%
sum = xs -> list.size xs == 0 ? 0 ; xs 0 + sum (list.tail 1 xs),
totals = [25, 140, 60]
%>Total: <% sum totals %>

Total: 225

The recursion stops at the empty list, because the right side of ; is evaluated only when it is needed. See Evaluation order.

Reusable helpers

Collect the helpers you use often in a first tag, or in a library document that every template imports (see Using a library):

<%
nl = "
",
bullets = items -> text.join nl (list.enum items => (i -> "- " + text.from i)),
plural = (n, word) -> text.from n + " " + word + (n == 1 ? "" ; "s")
%><% plural (3, "file") %> changed:
<% bullets ["README.md", "main.rs", "lib.rs"] %>

3 files changed:
- README.md
- main.rs
- lib.rs