[Booleans] another attempt (#831)

* another attempt at booleans

* Update about.md

like that?

---------

Co-authored-by: Colin Leach <[email protected]>

Author: colinleach

concepts/functions/.meta/config.json renamed to concepts.wip/functions/.meta/config.json

@@ -0,0 +1,9 @@
+{
+  "authors": [
+    "colinleach"
+  ],
+  "contributors": [
+    "BNAndras"
+  ],
+  "blurb": "Julia has boolean values true and false, operators ! (not), && (and), || (or)."
+}

concepts/functions/about.md renamed to concepts.wip/functions/about.md

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+# About
+
+## Booleans in Julia
+
+True or false values are represented by the `Bool` type.
+It contains only two values: `true` and `false`.
+
+```julia-repl
+julia> true
+true
+
+julia> false
+false
+
+julia> typeof(true)
+Bool
+```
+
+In contrast to several other languages, Julia deliberately has no concept of "truthiness", and only expressions which evaluate to `true` or `false` will be treated as a `Bool`.
+
+Specifically, empty arrays or strings will *not* be interpreted as `false`.
+There must be an appropriate test such as `isempty()` if you want special handling for empty values.
+
+## Boolean operators
+
+There are three [Boolean operators][boolean-operators] in Julia.
+
+`&&` is Boolean "and".
+It evaluates to `true` if the expressions on *both* sides of `&&` are `true`.
+
+```julia-repl
+julia> true && true
+true
+
+julia> true && false
+false
+```
+
+`||` is Boolean "or".
+It evaluates to `true` if an expression on *either* side of `||` is `true`.
+
+```julia-repl
+julia> true || true
+true
+
+julia> false || true
+true
+```
+
+`!` is Boolean "not".
+It exchanges (inverts) `true` and `false` values.
+
+```julia-repl
+julia> !true
+false
+
+julia> !false
+true
+```
+
+Similar operators will be conceptually familiar to users of many other languages, though the precise syntax may vary between languages.
+
+## Operator precedence
+
+In more complex expressions, it can be useful to know that `&&` has a slightly higher [precedence][operator-precedence] than `||` *(in the same way that `*` is applied before `+` in arithmetic expressions)*.
+
+Relying on this can be confusing and error-prone.
+For clarity, use parentheses to make your intention clear.
+
+## Short-circuit evaluation
+
+Does the expression `true || x` depend on the value of `x`, or can the compiler ignore `x`?
+
+Julia evaluates Boolean expressions from left to right, and stops when it has an unambiguous result.
+
+For example, `true || x` must be `true`, regardless of `x`, so `x` is not evaluated.
+Similarly, `false && y` is `false`, with no need to evaluate `y`.
+
+Conversely, `true && x` is `true` *only if* `x` is `true`, so `x` must be evaluated.
+
+Similarly, if we chain multiple operators:
+
+```julia-repl
+julia> true && false && something_else
+false
+```
+
+Because `true && false` must be `false`, the `something_else` is unimportant and is ignored by the compiler.
+
+In this case, `something_else` did not exist as a variable, but including it in this context gave no error *(test this in the REPL if you doubt it)*.
+
+Such short-circuit evaluation is quite often used by Julia programmers as a shortcut to trap runtime problems and edge cases:
+
+```julia
+all_ok || do_something()
+
+is_problem && do_something_else()
+```
+
+For example, the `do_something` might be an early `return` from the function if `all_ok` is `false`, or assigning a default value to a variable before continuing.
+
+This is a slight abuse of Boolean syntax, but it can be very convenient.
+
+## How Bools work internally
+
+If a Bool is included in an *arithmetic* expression, `true` is interpreted as `1` and `false` as `0`, reflecting how they are stored.
+
+If you are used to lower-level languages (C and similar), *please* avoid using this often.
+It will reduce code readability and make debugging harder.
+
+You may sometimes see the numerical values used as a quick way to count how many things are `true`.
+
+```julia-repl
+julia> true + false + true
+2
+```
+
+[operator-precedence]: https://docs.julialang.org/en/v1/manual/mathematical-operations/#Operator-Precedence-and-Associativity
+[boolean-operators]: https://docs.julialang.org/en/v1/manual/mathematical-operations/#Boolean-Operators

concepts/functions/introduction.md renamed to concepts.wip/functions/introduction.md

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+# Introduction
+
+## Booleans in Julia
+
+True or false values are represented by the `Bool` type.
+It contains only two values: `true` and `false`.
+
+```julia-repl
+julia> true
+true
+
+julia> false
+false
+```
+
+## Boolean Operators
+
+There are three Boolean operators in Julia.
+
+`&&` is Boolean "and".
+It evaluates to `true` if the expressions on *both* sides of `&&` are `true`.
+
+```julia-repl
+julia> true && true
+true
+
+julia> true && false
+false
+```
+
+`||` is Boolean "or".
+It evaluates to `true` if an expression on *either* side of `||` is `true`.
+
+```julia-repl
+julia> true || true
+true
+
+julia> false || true
+true
+```
+
+`!` is Boolean "not".
+It exchanges `true` and `false` values.
+
+```julia-repl
+julia> !true
+false
+
+julia> !false
+true
+```
+
+For longer and more complicated expressions, it is best to use parentheses to make your intention clear.
+
+```julia-repl
+julia> (true || false) && (false && true)
+false
+```

concepts/functions/links.json renamed to concepts.wip/functions/links.json

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+[
+  {
+    "url": "https://docs.julialang.org/en/v1/manual/mathematical-operations/#Boolean-Operators",
+    "description": "Boolean Operators, section in the manual"
+  }
+]

concepts/booleans/.meta/config.json

@@ -1091,8 +1091,8 @@
   "concepts": [
     {
       "uuid": "32384ed4-ef26-4118-8ea3-4d44c331e828",
-      "slug": "functions",
-      "name": "Functions"
+      "slug": "booleans",
+      "name": "Booleans"
     },
     {
       "uuid": "006ebce8-87cd-4695-87e6-8a7b8dc2f239",