Here are mnemonic “formulae” to help remember the relationship between eq, eql, equal, equalp, and = in Common Lisp:

• eq:
  

  eq: Exact same object in memory

  Think “Exact” for eq.

• eql:
  

  eql: eq + little more (for numbers and characters)

  Think “Little more” for eql.

• equal:
  

  equal: eql + structural comparison for lists, strings, etc.

  Think “All” for equal – checks all structural elements.

• equalp:
  

  equalp: equal + case-insensitive + number type agnostic

  Think “P for Practical” – it’s practical because it ignores case and number types.

• =:
  

  =: Numbers are equal regardless of type (float vs. integer)

  Think “= for Numbers” – only used for numeric comparison.

These mnemonics aim to encapsulate the essence of each function:

• eq is about memory identity.
• eql extends eq slightly for direct value comparison in some cases.
• equal checks for structural equivalence beyond what eql does.
• equalp adds case insensitivity and type flexibility to equal.
• = is strictly for numeric equality, ignoring type differences.

— Me@2024-12-14 10:57:53 AM

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2024.12.14 Saturday (c) All rights reserved by ACHK