# CONSIDER

Pattern matching construct that examines a value and executes different branches based on its structure.

## Syntax

```l4
CONSIDER expression
WHEN pattern1 THEN result1
WHEN pattern2 THEN result2
...
```

## Purpose

CONSIDER enables pattern matching on algebraic types (enums and records), lists, and other structured data. It's similar to `case` or `match` in other functional languages.

## Examples

**Example file:** 

```l4-file
-- CONSIDER keyword examples (pattern matching)

-- Matching Simple Enums

DECLARE Colour IS ONE OF red, green, blue

GIVEN c IS A Colour
`name of colour` c MEANS
  CONSIDER c
  WHEN red THEN "Red"
  WHEN green THEN "Green"
  WHEN blue THEN "Blue"

#EVAL `name of colour` red
#EVAL `name of colour` blue

-- Matching Lists

-- Sum all elements
GIVEN xs IS A LIST OF NUMBER
GIVETH A NUMBER
`sum of list` xs MEANS
  CONSIDER xs
  WHEN EMPTY THEN 0
  WHEN head FOLLOWED BY tail THEN head PLUS `sum of list` tail

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

-- Get first element
GIVEN ys IS A LIST OF NUMBER
GIVETH A NUMBER
`first element or zero` ys MEANS
  CONSIDER ys
  WHEN EMPTY THEN 0
  WHEN head FOLLOWED BY rest THEN head

#EVAL `first element or zero` (LIST 10, 20, 30)
#EVAL `first element or zero` EMPTY

-- Count elements
GIVEN a IS A TYPE, zs IS A LIST OF a
GIVETH A NUMBER
`length of list` zs MEANS
  CONSIDER zs
  WHEN EMPTY THEN 0
  WHEN hd FOLLOWED BY rest THEN 1 PLUS `length of list` rest

#EVAL `length of list` (LIST "a", "b", "c")

-- Matching MAYBE Values

GIVEN mx IS A MAYBE NUMBER
`get value or default to zero` mx MEANS
  CONSIDER mx
  WHEN NOTHING THEN 0
  WHEN JUST x THEN x

#EVAL `get value or default to zero` (JUST 42)
#EVAL `get value or default to zero` NOTHING

-- Matching Enums with Fields

DECLARE Shape IS ONE OF
  Circle HAS radius IS A NUMBER
  Rectangle HAS width IS A NUMBER, height IS A NUMBER
  Point

GIVEN s IS A Shape
`area of shape` s MEANS
  CONSIDER s
  WHEN Circle r THEN 3.14159 TIMES r TIMES r
  WHEN Rectangle w h THEN w TIMES h
  WHEN Point THEN 0

DECIDE `my circle` IS Circle WITH radius IS 5
DECIDE `my rectangle` IS Rectangle WITH width IS 4, height IS 6

#EVAL `area of shape` `my circle`
#EVAL `area of shape` `my rectangle`
#EVAL `area of shape` Point

-- Matching EITHER Values

GIVEN result IS AN EITHER STRING NUMBER
`handle either result` result MEANS
  CONSIDER result
  WHEN LEFT err THEN CONCAT "Error: ", err
  WHEN RIGHT val THEN CONCAT "Value: ", TOSTRING val

#EVAL `handle either result` (LEFT "not found")
#EVAL `handle either result` (RIGHT 42)

-- Matching Pairs/Tuples

DECLARE PairNum HAS first IS A NUMBER, second IS A NUMBER

GIVEN p IS A PairNum
`add pair values` p MEANS
  CONSIDER p
  WHEN PairNum a b THEN a PLUS b

DECIDE `first pair` IS PairNum WITH first IS 10, second IS 20
#EVAL `add pair values` `first pair`

-- Nested Pattern Matching

GIVEN maybeList IS A MAYBE (LIST OF NUMBER)
`sum of maybe list` maybeList MEANS
  CONSIDER maybeList
  WHEN NOTHING THEN 0
  WHEN JUST xs THEN `sum of list` xs

#EVAL `sum of maybe list` (JUST (LIST 1, 2, 3))
#EVAL `sum of maybe list` NOTHING
```



### Matching Enums

```l4
DECLARE Colour IS ONE OF red, green, blue

GIVEN c IS A Colour
colourName c MEANS
  CONSIDER c
  WHEN red THEN "Red"
  WHEN green THEN "Green"
  WHEN blue THEN "Blue"
```

### Matching Lists

```l4
GIVEN xs IS A LIST OF NUMBER
GIVETH A NUMBER
sumList xs MEANS
  CONSIDER xs
  WHEN EMPTY THEN 0
  WHEN head FOLLOWED BY tail THEN head PLUS sumList tail
```

### Matching MAYBE Values

```l4
GIVEN mx IS A MAYBE NUMBER
getOrDefault mx MEANS
  CONSIDER mx
  WHEN NOTHING THEN 0
  WHEN JUST x THEN x
```

### Matching Constructors with Fields

```l4
DECLARE Shape IS ONE OF
  Circle HAS radius IS A NUMBER
  Rectangle HAS width IS A NUMBER, height IS A NUMBER

GIVEN s IS A Shape
area s MEANS
  CONSIDER s
  WHEN Circle r THEN 3.14159 TIMES r TIMES r
  WHEN Rectangle w h THEN w TIMES h
```

### Using Wildcards

Use `_` to ignore values:

```l4
GIVEN xs IS A LIST OF NUMBER
firstElement xs MEANS
  CONSIDER xs
  WHEN EMPTY THEN 0
  WHEN head FOLLOWED BY _ THEN head
```

## OTHERWISE

Use OTHERWISE for a catch-all pattern:

```l4
GIVEN n IS A NUMBER
describe n MEANS
  CONSIDER n
  WHEN 0 THEN "zero"
  WHEN 1 THEN "one"
  OTHERWISE "many"
```

## Related Keywords

- **[IF](/l4/reference/control-flow/IF.md)** - Simple conditional alternative
- **[CONTROL-FLOW](/l4/reference/control-flow.md)** - All control flow keywords

> Note: WHEN, OTHERWISE, and BRANCH are part of the CONSIDER syntax.