LET

Introduces local bindings before an expression. The bindings are visible only within the IN clause.

Syntax

LET name IS expression IN body
LET name1 IS expr1, name2 IS expr2 IN body
LET
  name1 IS expression1
  name2 IS expression2
IN body

Purpose

LET allows you to:

Name intermediate values before using them
Break complex expressions into readable parts
Create local scope for temporary variables

Examples

Example file:

-- 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

Single Binding

DECIDE result IS
  LET x IS 5
  IN x TIMES 2

Multiple Bindings (Inline)

DECIDE result IS
  LET x IS 5, y IS 10
  IN x PLUS y

Multiple Bindings (Multi-line)

DECIDE result IS
  LET
    x IS 5
    y IS 10
    z IS 15
  IN x PLUS y PLUS z

Forward References

Bindings can reference earlier bindings:

DECIDE result IS
  LET
    x IS 5
    y IS x TIMES 2
    z IS y PLUS 3
  IN z

Binding Keywords

Multiple keywords can introduce bindings:

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

LET vs WHERE

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

-- Using LET (definitions before)
result1 MEANS LET x IS 1, y IS 2 IN x PLUS y

-- Using WHERE (definitions after)
result2 MEANS x PLUS y WHERE x MEANS 1, y MEANS 2

WHERE - Alternative local binding syntax
MEANS - Alternative assignment keyword

Note: IN, IS, and BE are part of the LET syntax, not separate keyword pages.

LET