# SI Units

This one-file module enforces Standard International unit types. This means that it will not allow a developer to add a time with a length or subtract a volume from a weight. It can also convert units from one prefix to another, i.e. km to cm.

The module has a structure, si-unit, that can only be accessed from within the module. It is used to store the information for the other derivative SI units (length, weight, temperature, etc.) The si-unit structure has quantity, prefix and suffix fields.

Type annotations are indicated by a double colon ("::") after the variable name, i.e. `variable::variable-type`

 SI Units

## Functions

### si-unit Structure

#### si-unit-quantity x::si-unit

Returns the quantity field value of the si-unit x.

#### si-unit-prefix x::si-unit

Returns the prefix field value of the si-unit x.

#### si-unit-suffix x::si-unit

Returns the suffix field value of the si-unit x.

### Makers/Constructors

#### make-si-length q::realunit::symbol

Creates an si-unit structure that has a quantity of q, a prefix of unit and a suffix of "m". When unit is `'nil`

### Math

The arguments for all math functions can be numbers or si-units or a mix of the two.

### Conversion

#### convert-to! x::si-unitnew-unit

Destructive version of `convert-to`.

## Examples

### Example 1

Marvin the Martian rover is measuring the distance from one rock to another. He can measure accurately down to 1 mm and up to 1 m.

Without using the SI module:

`(define (distance x1 y1 x2 y2) ; square root of ((x2 - x1) ^ 2 + (y2 - y1) ^ 2) (sqrt (+ (expt (- x2 x1) 2)(expt (- y2 y1) 2))))(distance 2.0 1.0 0.43 2.4) `

The process of conversion must be written in by hand otherwise the results are inaccurate. Also, how do we know that we're getting 2.4 m and not 2.4 mm?

Using the SI module:

`(define (distance x1 y1 x2 y2) ; square root of ((x2 - x1) ^ 2 + (y2 - y1) ^ 2) (sqrt (.+ (expt (.- x2 x1) 2)(expt (.- y2 y1) 2))))(distance (make-si-length 2.0 'nil)   ; 2.0 m (make-si-length 1.0 'centi) ; 1.0 cm = 0.01 m(make-si-length 0.43 'nil)  ; 0.43 m(make-si-length 2.4 'milli) ; 2.4 mm = 0.0024 m `

The difference is that the units are specified and that a different function is used for mathematical operations. Instead of `+`, we use `.+`