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A boolean has the type bool and can be either true or false. Common operations:

  • &&: logical and
  • ||: logical or
  • not: logical not. Note that ! is reserved for something else
  • <=, >=, <, >
  • ==: physical equal, compares data structures deeply: (1, 2) == (1, 2) is true. Convenient, but use with caution
  • ===: referential equal, compares shallowly. (1, 2) === (1, 2) is false. let myTuple = (1, 2); myTuple === myTuple is true.
  • !=: physical unequal
  • !==: referential unequal


Note: BuckleScript provides bindings to the JavaScript true and false, which aren't the same as the Reason/OCaml true and false! Don't use them interchangeably without proper conversion (Js.to_bool and Js.Boolean.to_js_boolean).

Tips & Tricks

Use physical equal tastefully. It's convenient, but might accidentally make you compare two deeply nested data structures and incur a big performance hit. It's also not always clear what counts as "equal". For example, is a piece of data foo equal to a lazy foo? Ideally, it'd have been pluggable. Future changes are coming to make this possible and reliable; if you're interested, check modular implicit.

Design Decisions

This section assumes knowledge with variants. If it's your first time reading this guide, feel free to come back to this later!

Boolean is (only conceptually) a special case of a variant: type bool = True | False. Design-wise, this elegantly removes the need to hard-code a boolean type in the type system. The drawback is that, just like variants, the constructors are compiled into a less readable but faster representation. This is why BuckleScript lacks information, at a later stages, to compile Reason true/false into JavaScript's true/false.