Further additions to Applicative

applicative/applicative.md

@@ -84,6 +84,60 @@ pure (+1) <*> [1..3]
 [(1,3),(1,4),(2,3),(2,4)]
 ```
 
+# Use cases[^4]
+
+```haskell
+Person
+  <$> parseString "name" o
+  <*> parseInt "age" o
+  <*> parseTelephone "telephone" o
+```
+
+Can also be written as
+
+```haskell
+liftA3 Person
+  (parseString "name" o)
+  (parseInt "age" o)
+  (parseTelephone "telephone" o)
+```
+[^4]: [FP Complete - Crash course to Applicative syntax](https://www.fpcomplete.com/haskell/tutorial/applicative-syntax/)
+
+# Use cases[^5]
+
+```haskell
+parsePerson :: Parser Person
+parsePerson = do
+  string "Name: "
+  name <- takeWhile (/= 'n')
+  endOfLine
+  string "Age: "
+  age <- decimal
+  endOfLine
+  pure $ Person name age
+```
+
+[^5]: [FP Complete - Crash course to Applicative syntax](https://www.fpcomplete.com/haskell/tutorial/applicative-syntax/)
+
+# Use cases[^6]
+
+```haskell
+helper :: () -> Text -> () -> () -> Int -> () -> Person
+helper () name () () age () = Person name age
+
+parsePerson :: Parser Person
+parsePerson = helper
+  <$> string "Name: "
+  <*> takeWhile (/= 'n')
+  <*> endOfLine
+  <*> string "Age: "
+  <*> decimal
+  <*> endOfLine
+```
+
+[^6]: [FP Complete - Crash course to Applicative syntax](https://www.fpcomplete.com/haskell/tutorial/applicative-syntax/)
+
+
 # Lifting
 
 - Seeing Functor as unary lifting and Applicative as n-ary lifting
@@ -122,6 +176,22 @@ instance Monoid a => Applicative ((,) a) where
   (u, f) <*> (v, x) = (u `mappend` v, f x)
 ```
 
+# Where are monoids again
+
+```haskell
+fmap (+1) ("blah", 0)
+("blah",1)
+
+("Woo", (+1)) <*> (" Hoo!", 0)
+("Woo Hoo!", 1)
+
+(,) <$> [1, 2] <*> [3, 4]
+[(1,3),(1,4),(2,3),(2,4)]
+
+liftA2 (,) [1, 2] [3, 4]
+[(1,3),(1,4),(2,3),(2,4)]
+```
+
 # Function apply
 
 - Applying a function to an `effectful` argument
@@ -155,6 +225,17 @@ pure f <*> pure x = pure (f x)
 u <*> pure y = pure ($ y) <*> u
 ```
 
+# Operators[^7]
+
+- `pure` wraps up a pure value into some kind of Applicative
+- `liftA2` applies a pure function to the values inside two `Applicative` wrapped values
+- `<$>` operator version of `fmap`
+- `<*>` apply a wrapped function to a wrapped value
+- `*>`, `<*`
+
+[^7]: [FP Complete - Crash course to Applicative syntax](https://www.fpcomplete.com/haskell/tutorial/applicative-syntax/)
+
+
 # Applicative vs monads
 
 - Applicative
@@ -162,7 +243,7 @@ u <*> pure y = pure ($ y) <*> u
   * Batching and aggregation
   * Concurrency/Independent
     - Parsing context free grammar
-    - Exploring all branches of computation (see `Alternative`)
+    - Exploring all branches of computation (see [`Alternative`](https://hackage.haskell.org/package/base-4.20.0.1/docs/Control-Applicative.html#t:Alternative))
 
 - Monads
   * Effects
@@ -183,6 +264,7 @@ u <*> pure y = pure ($ y) <*> u
 - [Applicative Programming with Effects](https://www.staff.city.ac.uk/~ross/papers/Applicative.html)
 - [optparse-applicative](https://hackage.haskell.org/package/optparse-applicative)
 - [Control Applicative](https://hackage.haskell.org/package/base-4.19.1.0/docs/Control-Applicative.html)
+- [Applicative functors for fun & parsing](https://arunraghavan.net/2018/02/applicative-functors-for-fun-and-parsing/)
 
 # Questions