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LET and WHERE

LET and WHERE give names to values that are used only inside one expression. LET puts the definitions before the expression; WHERE puts them after it. Use whichever matches the order of the legal text.

Syntax

LET name IS expression IN body

LET
  name1 IS expression1
  name2 IS expression2
IN body

body
WHERE
  name1 MEANS expression1
  name2 MEANS expression2

Several definitions go one per line, lined up in the same column; commas between them are a syntax error.

Example files:

-- LET keyword examples

-- Single Binding

DECIDE `result with single binding` IS
  LET x IS 5
  IN x TIMES 2

#EVAL `result with single binding`

-- Multiple Bindings (Inline)

DECIDE `sum of two values` IS
  LET
    x IS 5
    y IS 10
  IN x PLUS y

#EVAL `sum of two values`

-- Multiple Bindings (Multi-line)

DECIDE `sum of three values` IS
  LET
    x IS 5
    y IS 10
    z IS 15
  IN x PLUS y PLUS z

#EVAL `sum of three values`

-- Forward References

DECIDE `calculated result using forward references` IS
  LET
    `initial value` IS 5
    `doubled value` IS `initial value` TIMES 2
    `final value` IS `doubled value` PLUS 3
  IN `final value`

#EVAL `calculated result using forward references`

-- Various Binding Keywords

DECIDE `sum using various binding keywords` IS
  LET
    a IS 1
    b BE 2
    c MEAN 3
    d MEANS 4
    e = 5
  IN a PLUS b PLUS c PLUS d PLUS e

#EVAL `sum using various binding keywords`

-- Using = Operator

DECIDE `using equals for binding` IS
  LET
    val = 42
    result = val PLUS 8
  IN result

#EVAL `using equals for binding`

-- Nested LET Expressions

DECIDE `nested let expressions` IS
  LET `step one` IS 10
  IN LET `step two` IS `step one` TIMES 2
     IN LET `step three` IS `step two` PLUS 5
        IN `step three`

#EVAL `nested let expressions`

-- LET with Complex Expressions

GIVEN n IS A NUMBER
`factorial of` n MEANS
  IF n <= 1
  THEN 1
  ELSE LET `previous factorial` IS `factorial of` (n MINUS 1)
       IN n TIMES `previous factorial`

#EVAL `factorial of` 5

-- LET with Records

DECLARE Point HAS x IS A NUMBER, y IS A NUMBER

DECIDE `shifted point example` IS
  LET
    `the origin` IS Point WITH x IS 0, y IS 0
    `shifted point` IS Point WITH x IS `the origin`'s x PLUS 10, y IS `the origin`'s y PLUS 20
  IN `shifted point`

#EVAL `shifted point example`

-- LET with Lists

DECIDE `list of numbers example` IS
  LET
    `all numbers` IS LIST 1, 2, 3, 4, 5
  IN `all numbers`

#EVAL `list of numbers example`

-- LET in Function Definitions

GIVEN a IS A NUMBER
      b IS A NUMBER
      c IS A NUMBER
`quadratic discriminant` a b c MEANS
  LET
    `the discriminant` IS b TIMES b MINUS 4 TIMES a TIMES c
  IN `the discriminant`

#EVAL `quadratic discriminant` 1 5 6

,

-- 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)

LET

GIVEN price IS A NUMBER
GIVETH A NUMBER
`total price` price MEANS
  LET
    `the tax` IS price TIMES 0.2
    `the fee` IS 5
  IN price PLUS `the tax` PLUS `the fee`

After LET, a definition can use IS, BE, MEAN, MEANS or =:

DECIDE `the sum` IS
  LET
    a IS 1
    b BE 2
    c MEAN 3
    d MEANS 4
    e = 5
  IN a PLUS b PLUS c PLUS d PLUS e

WHERE

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

A definition under WHERE can take inputs, so it can be a local function, with or without its own GIVEN and GIVETH:

GIVEN a IS A NUMBER
      b IS A NUMBER
GIVETH A NUMBER
`sum of squares` a b MEANS
  `square of` a PLUS `square of` b
  WHERE
    GIVEN n IS A NUMBER
    GIVETH A NUMBER
    `square of` n MEANS n TIMES n

A WHERE can be nested inside a definition under another WHERE.

Scope

  • The names are visible only in the expression they belong to: after IN for LET, and in the body above for WHERE.
  • Definitions in one LET or one WHERE can refer to each other, in any order.
  • They can use the function's GIVEN inputs and any name defined outside.
  • A rule written as several clauses cannot be defined inside LET or WHERE.