syntax-help.md

Syntax Cheat Sheet

Comments

-- single line comment
{- multiline comment
-}

Basic types

True : Bool 
False : Bool

13 : number -- Int or Float depending on usage 
4.2 : Float

"hello world" : String 
'a' : Char

String manipulation

"hello" ++ " world" -- Concatenation
"age: " ++ (String.fromInt 42) -- Concatenation with numbers

Functions

add a b = a + b -- Definition
add 3 5 -- Example of use, returns 8

anotherAdd = (\a b -> a + b) -- Lambda / anonymous function 
anotherAdd 3 5 -- Returns 8

Lists

["elem1", "elem2"]
1 :: [2,3,4] -- Equals [1, 2, 3, 4]
List.map (\a -> a * 2) [1, 2, 3, 4] -- Equals [2, 4, 6, 8]

Tuples

("text", 42) -- Tuple with two values 
(34, 23, "text") -- Tuple with 3 values

More details about tuples

Conditions

myAge = 19
if myAge >= 18 then 
    "overage" 
else 
    "underage" 
-- Returns "overage" (this is an expression, not a statement)

Records

myUser = { username = "Marcus", password = "Secr3T" } 
myUser.username -- Returns "Marcus"

myUserWithChangedPassword = { myUser | password = "M0r3Secr3T" }
myUser.password -- Returns "Secr3T"
myUserWithChangedPassword.password -- Returns "M0r3Secr3T"

Type

-- Type alias
type alias User = { username : String, password : String }

-- Gives you a "constructor" function 'User'
User "Marcus" "Secr3T" == { username = "Marcus", password = "Secr3T" }


-- Custom type
type RemoteString = NotLoaded | Loading | Loaded String | OnError Int

{- For null values, you can use the Maybe custom type 
   that is either "Just something" or "Nothing":
-}
Just "value" : Maybe.Maybe String 
Nothing : Maybe.Maybe a

More informations about custom types

Cases

Match values against patterns

case myListOfString of
    [] -> -- Handles empty list
        "empty" 
        
    head :: others -> -- Store head of the list in head and the rest of the list in the List others
        head

case myStringMaybe of
    Just value -> 
        value 
    Nothing -> 
        "empty string"
        
case myNumber of
    0 -> 
        "the number is 0"
    1 -> 
        "the number is 1"
    _ -> 
        "the number is neither 0 nor 1"

Let Expressions

let these values be defined in this specific expression.

let 
    days =
        2 
    seconds =
        numberOfDays * 24 * 60 * 60 
in
(String.fromInt seconds) " seconds in " ++ (String.fromInt days) ++ " days"

Allows to split complex expressions into smaller definitions to ease the read. Use when needed but be careful, sometimes it is better to extract code into functions!

Modules

module MyModule exposing (..)

import List exposing (..) -- import module and expose everything
import List exposing ( map, foldl ) -- exposes only map and foldl

Qualified imports are preferred. Module names must match their file name, so module Parser.Utils needs to be in file Parser/Utils.elm .

Type Annotations

You can learn more about type annotations here.