# Expressions

_Expressions_ are the basis of programming in Ava. An expression is some code
that will produce a _value_ when evaluated.

## Syntax

Most non-[Definition](definitions.md) code is considered an expression.
Expressions include:

- Literals
- Variables
- Function Calls
- Control Expressions

In general, using a Literal or a Variable for an expression wraps that value in
a `pure` function that returns the value when evaluated.

## Control Expressions

### If Then

The `if/then` expression expects an expression that returns a _Boolean Value_.
That expression is evaluated. If it evaluates to `true`, the `then` expression
is evaluated. Otherwise the `else` expression is evaluated.

```
if <boolean expr> then <expr> else <expr>
```

### Do Yield

TODO: This is incomplete

The `do/yield` expression allows for imperative composition of monadic
expressions.

```
do
    x <- foo()
    y <- bar()
    _ <- baz()
yield
    x + y
```