Names

A sequence evaluates its steps in their written order. A step with a name tag gives its result to the later steps under that name. A reference reads a name back. This is the scope of let*: each binding sees the bindings before it.

The glyphs so far: 11, 2 new on this page

• 42Data: a value without a type
• number

  42
  A document: the tab is its type
• number

  42
  A dashed tab: the container declares the type
• add

  number

  1

  +

  number

  2
  Parts: a card inside a card
• kindA kind: a document of type kind
• OKDA rung: own, kind or default
• =3A port: the result of a document
• 0A sealed part: an evaluator cannot read it
• #NAME?An error value, in a red port
• numbera

  3
  A name tag: a binding for later stepsnew
• ref

  namesa
  A reference: a path from a root of the contextnew

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Event 0 of 18

Show how each card is drawnShow the JSONShow the referencesOpen in the playground

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seq

1. 1
   adda

   number

   1

   +

   number

   2
2. 2
   multiplyb

   ref

   namesa

   ×

   number

   10
3. 3
   ▸log

   ref

   namesb

Select Next, or select a card, to see how its evaluation works.

The world: the effects, in orderNo effect yet.

The result shows at the end of the evaluation.

Step 1 gives a. Step 2 reads a, and gives b. Step 3 reads b and logs it. Select each step, and look at its names in the panel.

The context grows

Select the steps one after the other, and look at the panel. Step 1 can read no name. Step 2 can read a. Step 3 can read a and b. The sequence makes the context larger after each named step. A reference to a later name gives #NAME?, because that name does not exist yet.

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Event 4 of 4

Show how each card is drawnShow the JSONOpen in the playground

The evaluation of seq ends with the error #VALUE!: The value of ref is a path: a list that starts with a root, for example ["names", "a"].

seq

1. 1
   ref

   no path

   #VALUE!
2. 2
   numberlater

   1

   not evaluated

#VALUE!

The result of the document: #VALUE!

Step 1 reads later, and step 2 gives later. The name does not exist when step 1 starts, thus the result is #NAME?.

A name and a nesting are two forms of one thing

A sequence with names is the same as nested bindings: let a = 1 + 2 in let b = a * 10 in log b. The sequence writes the nesting as a list, so that a person reads it from the top to the bottom.