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Add docs about the REPL
0c3601eEvan Czaplicki
docs/repl.mdnew file+66
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1+# `elm repl`
2+
3+The REPL lets you interact with Elm values and functions in your terminal.
4+
5+<br>
6+
7+## Install
8+
9+You need [Elm][install] and [node.js][node] for the REPL. Elm generates JavaScript and we currently use node.js to evaluate it!
10+
11+[install]: https://guide.elm-lang.org/install.html
12+[node]: https://nodejs.org/en/download/
13+
14+<br>
15+
16+## Use
17+
18+You can type in expressions, definitions, union types, and module imports using normal Elm syntax.
19+
20+```elm
21+> 1 + 1
22+2 : number
23+
24+> "hello" ++ "world"
25+"helloworld" : String
26+```
27+
28+The same can be done with definitions and union types:
29+
30+```elm
31+> fortyTwo = 42
32+42 : number
33+
34+> increment n = n + 1
35+<function> : number -> number
36+
37+> increment 41
38+42 : number
39+
40+> factorial n = if n < 1 then 1 else n * factorial (n-1)
41+<function> : number -> number
42+
43+> factorial 5
44+120 : number
45+
46+> type Either a b = Left a | Right b
47+
48+> case Left 32 of \
49+| Left n -> 2 * n \
50+| Right m -> m + 1
51+64 : number
52+```
53+
54+When you run `elm repl` in a project with an `elm.json` file, you can import any module available in the project:
55+
56+```elm
57+> import String
58+
59+> String.length "hello"
60+5 : Int
61+
62+> String.reverse "flow"
63+"wolf" : String
64+```
65+
66+Core modules, package modules, your modules. Whatever is available! So if you create a module named `MyThing` in your project, you can say `import MyThing` in the REPL and mess with anything it exposes.