WIP draft of tuples concept

concepts.wip/tuples/.meta/config.json

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+{
+  "authors": [
+    "colinleach"
+  ],
+  "contributors": [],
+  "blurb": "Tuples are similar to arrays, but immutable. The individual entries can have names."
+}

concepts.wip/tuples/about.md

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+# About
+
+A [`Tuple`][tuple] is quite similar to a [`Vector`][vector] (1-D `array`) in many ways.
+However, there are a few important differences.
+
+- Tuples are written in parentheses `( )` instead of brackets `[ ]`, though the parentheses can be omitted in cases where the meaning is unambiguous.
+- Tuples are not required to be homogeneous: the type of each element is stored independently.
+- Tuples are immutable.
+
+```julia-repl
+julia> t = (3, 5.2, "xyz")
+(3, 5.2, "xyz")
+
+# Non-homogeneous:
+julia> dump(t)
+Tuple{Int64, Float64, String}
+  1: Int64 3
+  2: Float64 5.2
+  3: String "xyz"
+
+julia> t[2]
+5.2
+
+# Immutable:
+julia> t[2] = 7.3
+ERROR: MethodError: no method matching setindex!(::Tuple{Int64, Float64, String}
+```
+
+For any tuple, it is possible to access elements by index or loop over them.
+
+Any purely numerical tuple can also be used in mathematical functions such as `sum()`, exactly like arrays.
+
+## Named tuples
+
+A [`Tuple`][tuple] contains only values.
+A [`NamedTuple`][namedtuple] pairs each value with a name.
+
+Individual fields can then be accessed with "dot" notation:
+
+```jullia-repl
+julia> nt = (a = 1, b = 7.3)
+(a = 1, b = 7.3)
+
+julia> dump(nt)
+@NamedTuple{a::Int64, b::Float64}
+  a: Int64 1
+  b: Float64 7.3
+
+julia> nt.b
+7.3
+```
+
+The name is stored internally as a [`Symbol`][symbol] (discussed in a later Concept), so there is an alternative syntax available:
+
+```julia-repl
+julia> nt[:b]
+7.3
+```
+
+It is sometimes useful to split creation of a named tuple into two steps:
+
+1. Define the fields as as custom data type.
+2. Use this as a constructor to convert a tuple to a named tuple.
+
+```julia-repl
+# Step 1
+julia> Student = NamedTuple{(:name, :grade), Tuple{String, Int}}
+@NamedTuple{name::String, grade::Int64}
+
+julia> typeof(Student)
+DataType
+
+# Step 2
+julia> s = Student( ("Helen", 3) )
+(name = "Helen", grade = 3)
+
+julia> dump(s)
+@NamedTuple{name::String, grade::Int64}
+  name: String "Helen"
+  grade: Int64 3
+```
+
+There are surprisingly many other ways to create a named tuple, so check the [documentation][namedtuple] if you are interested.
+
+It is easy to get either the names (also called "keys") or values separately:
+
+```julia-repl
+julia> keys(s)
+(:name, :grade)
+
+julia> values(s)
+("Helen", 3)
+```
+
+Note that iteration over a named tuple only produces the _values_, and the _names_ are ignored.
+
+If the names are also useful in an iteration, more steps are needed.
+The `NamedTuple` can be converted to a [`Vector`][vector] of [`Pairs`][pairs] (`Pair`s are the subject of another Concept).
+
+```julia-repl
+julia> pairs(s)
+pairs(::NamedTuple) with 2 entries:
+  :name  => "Helen"
+  :grade => 3
+
+julia> collect(pairs(s))
+2-element Vector{Pair{Symbol, Any}}:
+  :name => "Helen"
+  :grade => 3
+```
+
+The named tuple is thus quite dictionary-like.
+However, it differs from a [`Dict`][dict] in Julia in some important ways:
+
+- A `Dict` is mutable; a `NamedTuple`, like any tuple, is immutable.
+- A `Dict` is flexible in the type of keys it accepts; a `NamedTuple` can only use symbols as keys.
+
+Tuples (named or not) are a very common way to transfer structured data to and from functions.
+Immutability is good in this use case, providing data safety and allowing more aggressive compiler optimizations.
+
+
+[tuple]: https://docs.julialang.org/en/v1/base/base/#Core.Tuple
+[namedtuple]: https://docs.julialang.org/en/v1/base/base/#Core.NamedTuple
+[vector]: https://docs.julialang.org/en/v1/base/arrays/#Base.Vector
+[pairs]: https://docs.julialang.org/en/v1/base/collections/#Core.Pair
+[symbol]: https://docs.julialang.org/en/v1/base/base/#Core.Symbol

concepts.wip/tuples/introduction.md

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+# Introduction

concepts.wip/tuples/links.json

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+[
+  {
+    "url": "https://docs.julialang.org/en/v1/base/base/#Core.Tuple",
+    "description": "Tuple section in the manual."
+  },
+  {
+    "url": "https://docs.julialang.org/en/v1/base/base/#Core.NamedTuple",
+    "description": "NamedTuple section in the manual."
+  }
+]