4. Values and expressions

A document can say a thing once and use it everywhere: the issue's date in the heading and the notice, a fee in the table and in the text. This chapter shows how to insert values from the frontmatter, how to compute and format them, and how a param can follow a value. The newsletter's dates, fees and names now come from its frontmatter.

The newsletter, with values#

---
md:
  version: 1
title: Elm Street Garden
issue: November 2026
editor: {name: Maya Okafor, phone: 555-0142}
workday: "2026-11-07"
water_off: "2026-11-15"
fees: {small: 25, large: 40}
plots: {small: 12, large: 8, taken: 17}
helpers: [Maya, Tomás, Priya, Jun]
---

# {{md.doc.title}}

News from the plots, {{md.doc.issue}}, from {{md.doc.editor}}.

::: box {padding=m border-radius=m background=(md.daysBetween(md.today(), md.doc.water_off) <= 14 ? "accent" : "subtle")}

**The water goes off on :value[November 15, 2026]{of=(md.date(md.doc.water_off, "long"))}** for the winter. Empty your cans and hoses before then.

::: /box

## Work day

:value[Saturday, November 7, 2026]{of=(md.date(md.doc.workday, "full"))}, from nine to noon, with :value[Maya, Tomás, Priya, and Jun]{of=(md.list(md.doc.helpers))} leading the jobs.

## Plots for next year

| Size | Plots | Fee for the year |
| --- | ---: | ---: |
| Small | {{md.doc.plots.small}} | :value[$25.00]{of=(md.currency(md.doc.fees.small, "USD"))} |
| Large | {{md.doc.plots.large}} | :value[$40.00]{of=(md.currency(md.doc.fees.large, "USD"))} |

:value[3 plots are]{of=(string(md.doc.plots.small + md.doc.plots.large - md.doc.plots.taken) + " " + md.plural(md.doc.plots.small + md.doc.plots.large - md.doc.plots.taken, "plot is", "plots are"))} still free. Call {{md.doc.editor.name}} at {{md.doc.editor.phone}} to take one.

Try it in the playground.

The frontmatter's values#

The frontmatter is YAML, and a value can be more than a line of text:

Quote a value that would otherwise read as something else: a time such as "9:30", or a version such as "1.10", which unquoted is the number 1.1.

Inserting a value#

{{…}} inserts a value where it stands. Inside it goes a path: where the value is, written as names joined by dots.

• md.doc is your frontmatter, so {{md.doc.issue}} is the issue key: November 2026.
• A dot goes into a record: {{md.doc.plots.small}} is 12, and {{md.doc.editor.phone}} is 555-0142.
• A record with a name reads as its name, since it stands for someone or something: {{md.doc.editor}} is Maya Okafor.
• md.lang is the document's language, en unless the frontmatter says otherwise (below).
• A number is written as it is (25, 12.5), and a date as you wrote it (2026-11-07). To write them for readers, $25.00 or November 7, use value, below.

true and false are written as they are, and an empty value (note: with nothing after it) as nothing. A list isn't inserted as it is, since there are many ways to write one; {{md.doc.helpers}} is a diagnostic (F024), and so is a record without a name. Write it with md.list, below, or one item at a time with each (chapter 5).

{{…}} works in text, in headings, in table cells, in a link's address ([the rules](https://example.org/{{md.doc.rules}})), and in params written in quotes (title="Plot {{md.doc.plot}}"). It doesn't work in code, which always shows what you wrote.

A path that doesn't exist, such as {{md.doc.isue}}, is a diagnostic (F014) that suggests the nearest one, here md.doc.issue. To write double braces as text, start them with a backslash: \{{like this}}.

Computed text: value#

A path only finds a value; it can't do anything to it. For text that is computed or formatted, use the inline component value. Its of param is the text to show, and its brackets hold the fallback, the text a plain renderer shows instead:

---
fees: {small: 25, large: 40}
---

A small plot is :value[$25.00]{of=(md.currency(md.doc.fees.small, "USD"))} a year.

The fallback should be what the value usually is, so the text still reads well on GitHub. It doesn't change when the frontmatter does, so it's a sample, not a promise.

Expressions#

The (…) in of=(…) is an expression: a small formula, computed when the page is made. Any param can be one, not only of. In the notice above, background is computed: the accent color when the water goes off within two weeks, the subtle one before.

Expressions are written in CEL, a small language for formulas. You'll need only a little of it:

Text in an expression is in double quotes, or single ones: 'subtle'. Paths work as in {{…}}: md.doc.fees.small is the small plots' fee. Parentheses group, as in arithmetic: (md.doc.plots.small + md.doc.plots.large) * 2. An expression is written on one line.

value shows text, so a number goes through string(…) or one of the formatting functions below first: of=(string(size(md.doc.helpers))), not of=(size(md.doc.helpers)).

Text has a few functions of its own, written after it with a dot:

Keys that may be missing#

A path to a key the frontmatter doesn't have is a mistake, in {{…}} and in expressions alike. When a key is there in some issues and not in others, ask first with has, and read it only when it's there:

---
title: Elm Street Garden
---

# :value[Elm Street Garden]{of=(has(md.doc.subtitle) ? md.doc.title + ": " + md.doc.subtitle : md.doc.title)}

Numbers, text and true or false#

An expression knows what kind of value each part is, and checks that they fit before the page is made:

• Text and numbers don't mix. "Plot " + 4 is a mistake: write "Plot " + string(4). Comparing them is one too.
• Whole numbers. A number written in an expression without a decimal point, 4, and what size(…) and md.loop give, are whole numbers, and dividing two of them drops the remainder: size(md.doc.helpers) / 3 is 1 with four helpers. Write double(size(md.doc.helpers)) / 3 for 1.333…. A number from the frontmatter is never whole in this sense, so md.doc.fees.small / 2 is 12.5. A list's place must be a whole number, and int(…) makes one: md.doc.helpers[int(md.doc.plots.taken) % 4].
• Dates are text, written year, month and day, so they compare as dates do: md.doc.workday < md.doc.water_off is true.
• true and false are what comparisons give, and what when takes in chapter 5. if and ? take nothing else: a number or a text is a mistake there, never a stand-in for true.

When an expression is wrong#

A mistake in an expression is a diagnostic (F024) that points to the place in it, and says what is wrong:

string, double and int are the expression's words for text, a number and a whole number. Where the mistake is, the component shows its fallback, or the param is left out as if you hadn't written it, so the rest of the page is still there.

{{…}} takes only a path, never a formula: {{ md.doc.fees.small * 4 }} shows as written, with a warning (W106) that suggests value.

Formatting for readers#

CompMD's own functions start with md.. These format values for readers:

And these work with dates:

Chapter 8 has the rest, for dates, numbers and lists.

The document's language decides how they write: with lang: de under md in the frontmatter, md.date(md.doc.workday, "long") gives 7. November 2026, and md.currency(25, "EUR") gives 25,00 €.

Next: showing and repeating, for parts that show only sometimes, and lists made from the frontmatter.