diff options
| -rw-r--r-- | elm.html.markdown | 31 |
1 files changed, 21 insertions, 10 deletions
diff --git a/elm.html.markdown b/elm.html.markdown index f395e85b..944ab770 100644 --- a/elm.html.markdown +++ b/elm.html.markdown @@ -74,6 +74,10 @@ List.head [] -- Nothing fst ("elm", 42) -- "elm" snd ("elm", 42) -- 42 +-- The empty tuple, or "unit", is sometimes used as a placeholder. +-- It is the only value of its type, also called "Unit". +() + -- Records are like tuples but the fields have names. The order of fields -- doesn't matter. Notice that record values use equals signs, not colons. { x = 3, y = 7 } @@ -116,6 +120,7 @@ case aList of x::xs -> "matches a list of at least one item whose head is " ++ toString x -- Pattern matches go in order. If we put [x] last, it would never match because -- x::xs also matches (xs would be the empty list). Matches do not "fall through". +-- The compiler will alert you to missing or extra cases. -- Pattern match on a Maybe. case List.head aList of @@ -226,11 +231,13 @@ origin = { x = 0, y = 0, z = 0 } -- You can give existing types a nice name with a type alias. -type alias Point3D = { x : Float, y : Float, z : Float } +type alias Point3D = + { x : Float, y : Float, z : Float } -- If you alias a record, you can use the name as a constructor function. otherOrigin : Point3D -otherOrigin = Point3D 0 0 0 +otherOrigin = + Point3D 0 0 0 -- But it's still the same type, so you can equate them. origin == otherOrigin -- True @@ -238,23 +245,27 @@ origin == otherOrigin -- True -- By contrast, defining a union type creates a type that didn't exist before. -- A union type is so called because it can be one of many possibilities. -- Each of the possibilities is represented as a "tag". -type Direction = North | South | East | West +type Direction = + North | South | East | West -- Tags can carry other values of known type. This can work recursively. -type IntTree = Leaf | Node Int IntTree IntTree +type IntTree = + Leaf | Node Int IntTree IntTree -- "Leaf" and "Node" are the tags. Everything following a tag is a type. -- Tags can be used as values or functions. root : IntTree -root = Node 7 Leaf Leaf +root = + Node 7 Leaf Leaf -- Union types (and type aliases) can use type variables. -type Tree a = Leaf | Node a (Tree a) (Tree a) --- "The type tree of a is a leaf, or a node of a, tree of a, and tree of a." +type Tree a = + Leaf | Node a (Tree a) (Tree a) +-- "The type tree-of-a is a leaf, or a node of a, tree-of-a, and tree-of-a." --- You can pattern match union tags. The uppercase tags must be matched exactly. --- The lowercase variables will match anything. Underscore also matches --- anything, but signifies that you aren't using it. +-- Pattern match union tags. The uppercase tags will be matched exactly. The +-- lowercase variables will match anything. Underscore also matches anything, +-- but signifies that you aren't using it. leftmostElement : Tree a -> Maybe a leftmostElement tree = case tree of |