# WHERE

Introduces local definitions within an expression. Allows defining helper values or functions that are only visible within the enclosing scope.

## Syntax

```l4
expression
WHERE
  name1 MEANS expression1
  name2 MEANS expression2
```

## Purpose

WHERE clauses let you:

1. Break complex expressions into named parts
2. Avoid repetition by naming common subexpressions
3. Create helper functions local to a definition

## Examples

**Example file:** 

```l4-file
-- WHERE keyword examples

-- Basic Local Definitions

GIVEN radius IS A NUMBER
`area of circle` radius MEANS
  pi TIMES radius TIMES radius
  WHERE
    pi MEANS 3.14159

#EVAL `area of circle` 10

-- Multiple Local Definitions

GIVEN x IS A NUMBER, y IS A NUMBER
`sum times difference` x y MEANS
  `sum value` TIMES `diff value`
  WHERE
    `sum value` MEANS x PLUS y
    `diff value` MEANS x MINUS y

#EVAL `sum times difference` 10 3

-- Avoiding Repetition

GIVEN a IS A NUMBER
      b IS A NUMBER
      c IS A NUMBER
`quadratic discriminant` a b c MEANS
  `b squared` MINUS `four a c`
  WHERE
    `b squared` MEANS b TIMES b
    `four a c` MEANS 4 TIMES a TIMES c

#EVAL `quadratic discriminant` 1 5 6

-- Nested WHERE Clauses

outer MEANS
  middle
  WHERE
    middle MEANS
      inner
      WHERE
        inner MEANS 42

#EVAL outer

-- Local Functions

GIVEN n1 IS A NUMBER, n2 IS A NUMBER
`sum of squares` n1 n2 MEANS
  `square of` n1 PLUS `square of` n2
  WHERE
    `square of` n MEANS n TIMES n

#EVAL `sum of squares` 3 4

-- Complex Example: Pythagorean Distance

GIVEN x1 IS A NUMBER
      y1 IS A NUMBER
      x2 IS A NUMBER
      y2 IS A NUMBER
`distance between points` x1 y1 x2 y2 MEANS
  `square root` (`dx squared` PLUS `dy squared`)
  WHERE
    `delta x` MEANS x2 MINUS x1
    `delta y` MEANS y2 MINUS y1
    `dx squared` MEANS `delta x` TIMES `delta x`
    `dy squared` MEANS `delta y` TIMES `delta y`
    `square root` n MEANS sqrt n

IMPORT math

#EVAL `distance between points` 0 0 3 4

-- WHERE with Pattern Matching

GIVEN xs IS A LIST OF NUMBER
GIVETH A NUMBER
`sum of list using where` xs MEANS
  CONSIDER xs
  WHEN EMPTY THEN 0
  WHEN head FOLLOWED BY tail THEN head PLUS `rest sum`
    WHERE
      `rest sum` MEANS `sum of list using where` tail

#EVAL `sum of list using where` (LIST 1, 2, 3, 4, 5)
```



### Basic Local Definitions

```l4
circleArea radius MEANS
  pi TIMES radius TIMES radius
  WHERE
    pi MEANS 3.14159
```

### Multiple Local Definitions

```l4
calculation x y MEANS
  sum TIMES diff
  WHERE
    sum MEANS x PLUS y
    diff MEANS x MINUS y
```

### Nested WHERE Clauses

```l4
outer MEANS
  middle
  WHERE
    middle MEANS
      inner
      WHERE
        inner MEANS 42
```

### Local Functions

```l4
sumSquares x y MEANS
  square x PLUS square y
  WHERE
    square n MEANS n TIMES n
```

## WHERE vs LET

- **WHERE** - Definitions come after the main expression
- **LET** - Definitions come before the main expression

```l4
-- Using WHERE (definitions after)
result1 MEANS x PLUS y WHERE x MEANS 1, y MEANS 2

-- Using LET (definitions before)
result2 MEANS LET x IS 1, y IS 2 IN x PLUS y
```

## Related Keywords

- **[LET](/l4/reference/functions/LET.md)** - Alternative local binding syntax
- **[MEANS](/l4/reference/functions/MEANS.md)** - Defines the binding

## See Also

- **[Syntax: Local Declarations](/l4/reference/syntax.md)** - Scope rules