diff options
| -rw-r--r-- | haskell.html.markdown | 48 |
1 files changed, 29 insertions, 19 deletions
diff --git a/haskell.html.markdown b/haskell.html.markdown index f3b84bdd..1cc79ec9 100644 --- a/haskell.html.markdown +++ b/haskell.html.markdown @@ -330,15 +330,19 @@ distance (Point x y) (Point x' y') = sqrt $ dx + dy -- Types can have multiple data constructors with arguments, too -data Name = Mononym String | FirstLastName String String | FullName String String String +data Name = Mononym String + | FirstLastName String String + | FullName String String String -- To make things clearer we can use record syntax -data Point2D = CartesianPoint2D { x :: Float, y :: Float } | PolarPoint2D { r :: Float, theta :: Float } +data Point2D = CartesianPoint2D { x :: Float, y :: Float } + | PolarPoint2D { r :: Float, theta :: Float } myPoint = CartesianPoint2D { x = 7.0, y = 10.0 } --- Using record syntax automatically creates accessor functions (the name of the field) +-- Using record syntax automatically creates accessor functions +-- (the name of the field) xOfMyPoint = x myPoint @@ -357,8 +361,9 @@ myPoint'2 = CartesianPoint2D 3.3 4.0 -- It's also useful to pattern match data constructors in `case` expressions -distanceFromOrigin x = case x of (CartesianPoint2D x y) -> sqrt $ x ** 2 + y ** 2 - (PolarPoint2D r _) -> r +distanceFromOrigin x = + case x of (CartesianPoint2D x y) -> sqrt $ x ** 2 + y ** 2 + (PolarPoint2D r _) -> r -- Your data types can have type parameters too: @@ -386,8 +391,9 @@ somePerson :: Person someCircle :: Circle distance :: Point -> Point -> Float --- The following would compile and run without issue, even though it does not make --- sense semantically, because the type synonyms reduce to the same base types +-- The following would compile and run without issue, +-- even though it does not make sense semantically, +-- because the type synonyms reduce to the same base types distance (getMyHeightAndWeight somePerson) (findCenter someCircle) @@ -397,10 +403,11 @@ distance (getMyHeightAndWeight somePerson) (findCenter someCircle) -- Typeclasses are one way Haskell does polymorphism -- They are similar to interfaces in other languages --- A typeclass defines a set of functions that must work on any type that is in --- that typeclass. +-- A typeclass defines a set of functions that must +-- work on any type that is in that typeclass. --- The Eq typeclass is for types whose instances can be tested for equality with one another +-- The Eq typeclass is for types whose instances can +-- be tested for equality with one another. class Eq a where (==) :: a -> a -> Bool @@ -428,27 +435,30 @@ canProceedThrough t = t /= Red -- You can NOT create an instance definition for a type synonym --- Functions can be written to take typeclasses with type parameters, rather than types, --- assuming that the function only relies on features of the typeclass +-- Functions can be written to take typeclasses with type parameters, +-- rather than types, assuming that the function only relies on +-- features of the typeclass isEqual (Eq a) => a -> a -> Bool isEqual x y = x == y --- Note that x and y MUST be the same type, as they are both defined as being of type parameter 'a' --- A typeclass does state that different types in the typeclass can be mixed together --- So `isEqual Red 2` is invalid, even though 2 is an Int which is an instance of Eq, and Red is --- a TrafficLight which is also an instance of Eq +-- Note that x and y MUST be the same type, as they are both defined +-- as being of type parameter 'a'. +-- A typeclass does not state that different types in the typeclass can +-- be mixed together. +-- So `isEqual Red 2` is invalid, even though 2 is an Int which is an +-- instance of Eq, and Red is a TrafficLight which is also an instance of Eq -- Other common typeclasses are: -- Ord for types that can be ordered, allowing you to use >, <=, etc. -- Read for types that can be created from a string representation -- Show for types that can be converted to a string for display --- Num, Real, Integral, Fractional for types that can do mathematical calculation +-- Num, Real, Integral, Fractional for types that can do math -- Enum for types that can be stepped through -- Bounded for types with a maximum and minimum --- Haskell can automatically make types part of Eq, Ord, Read, Show, Enum, and Bounded --- with the `deriving` keyword at the end of the type declaration +-- Haskell can automatically make types part of Eq, Ord, Read, Show, Enum, +-- and Bounded with the `deriving` keyword at the end of the type declaration data Point = Point Float Float deriving (Eq, Read, Show) |