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WHERE

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

Syntax

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:

-- 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

circleArea radius MEANS
  pi TIMES radius TIMES radius
  WHERE
    pi MEANS 3.14159

Multiple Local Definitions

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

Nested WHERE Clauses

outer MEANS
  middle
  WHERE
    middle MEANS
      inner
      WHERE
        inner MEANS 42

Local Functions

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
-- 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
  • LET - Alternative local binding syntax
  • MEANS - Defines the binding

See Also