nil in Smalltalk

An object like any other; ifNil:ifNotNil: branches on it.

"nil is an object too; ifNil:ifNotNil: replaces null checks."
| proofreader assistant |
proofreader := 'Alba'.
assistant := nil.

proofreader
    ifNil: [Transcript show: 'Nobody reading proofs'; cr]
    ifNotNil: [:who | Transcript show: who, ' reads proofs'; cr].

assistant
    ifNil: [Transcript show: 'No assistant on shift'; cr].

Transcript show: (assistant isNil) printString; cr.
Transcript show: (assistant ifNil: ['(unassigned)']) ; cr.

How it works

1. ifNotNil: passes the non-nil value into its block.
2. ifNil: supplies the default for the missing case.
3. isNil answers the plain boolean question.

Keywords and builtins used here

• Transcript
• assistant
• cr
• isNil
• nil
• printString
• proofreader

The run, in numbers

Lines

14

Characters to type

435

Tokens

77

Three-star pace

60 tpm

At the three-star pace of 60 tokens a minute, this run takes about 77 seconds.

Type this snippet

Step 3 of 3 in Control flow, step 9 of 31 in Language basics.

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