From 45281224fa78b2a178957b6ac9e0fffb8e1acd9f Mon Sep 17 00:00:00 2001 From: Emiel van de Laar Date: Mon, 22 Mar 2021 17:49:10 +0100 Subject: [PATCH] Update various examples * `let` no longer required in PSCi * Properly emit output, `(Just 1)` vs `Just (1)` * Remove `sumTwo` as it is too convoluted of an example --- purescript.html.markdown | 86 ++++++++++++++++++++-------------------- 1 file changed, 42 insertions(+), 44 deletions(-) diff --git a/purescript.html.markdown b/purescript.html.markdown index eff8fb43..c848c2a4 100644 --- a/purescript.html.markdown +++ b/purescript.html.markdown @@ -84,12 +84,12 @@ world""" -- "Hello\nworld" -- and purescript-maybe (Data.Maybe) -- Safe access return Maybe a -head [1,2,3] -- Just (1) -tail [3,2,1] -- Just ([2,1]) -init [1,2,3] -- Just ([1,2]) -last [3,2,1] -- Just (1) +head [1,2,3] -- (Just 1) +tail [3,2,1] -- (Just [2,1]) +init [1,2,3] -- (Just [1,2]) +last [3,2,1] -- (Just 1) -- Array access - indexing -[3,4,5,6,7] !! 2 -- Just (5) +[3,4,5,6,7] !! 2 -- (Just 5) -- Range 1..5 -- [1,2,3,4,5] length [2,2,2] -- 3 @@ -100,29 +100,28 @@ append [1,2,3] [4,5,6] -- [1,2,3,4,5,6] -- -- 3. Records are JavaScript objects, with zero or more fields, which -- can have different types. --- In psci you have to write `let` in front of the function to get a --- top level binding. -let book = {title: "Foucault's pendulum", author: "Umberto Eco"} +book = {title: "Foucault's pendulum", author: "Umberto Eco"} -- Access properties book.title -- "Foucault's pendulum" -let getTitle b = b.title +getTitle b = b.title -- Works on all records with a title (but doesn't require any other field) getTitle book -- "Foucault's pendulum" getTitle {title: "Weekend in Monaco", artist: "The Rippingtons"} -- "Weekend in Monaco" -- Can use underscores as shorthand _.title book -- "Foucault's pendulum" -- Update a record -let changeTitle b t = b {title = t} +changeTitle b t = b {title = t} getTitle (changeTitle book "Ill nome della rosa") -- "Ill nome della rosa" -- -- 4. Functions --- In psci's multiline mode -let sumOfSquares :: Int -> Int -> Int - sumOfSquares x y = x*x + y*y +-- In PSCi's paste mode +sumOfSquares :: Int -> Int -> Int +sumOfSquares x y = x*x + y*y sumOfSquares 3 4 -- 25 -let myMod x y = x % y + +myMod x y = x % y myMod 3.0 2.0 -- 1.0 -- Infix application of function 3 `mod` 2 -- 1 @@ -132,48 +131,47 @@ myMod 3.0 2.0 -- 1.0 sumOfSquares 3 4 * sumOfSquares 4 5 -- 1025 -- Conditional -let abs' n = if n>=0 then n else -n +abs' n = if n>=0 then n else -n abs' (-3) -- 3 -- Guarded equations -let abs'' n | n >= 0 = n - | otherwise = -n +-- In PSCi's paste mode +abs'' n | n >= 0 = n + | otherwise = -n -- Pattern matching -- Note the type signature, input is a list of numbers. The pattern matching -- destructures and binds the list into parts. --- Requires purescript-lists (Data.List) -let first :: forall a. List a -> a - first (Cons x _) = x -first (toList [3,4,5]) -- 3 -let second :: forall a. List a -> a - second (Cons _ (Cons y _)) = y -second (toList [3,4,5]) -- 4 -let sumTwo :: List Int -> List Int - sumTwo (Cons x (Cons y rest)) = x + y : rest -fromList (sumTwo (toList [2,3,4,5,6])) :: Array Int -- [5,4,5,6] +-- Requires purescript-lists (Data.List) and purescript-maybe (Data.Maybe) +first :: forall a. List a -> Maybe a +first (x : _) = Just x +first Nil = Nothing +first (fromFoldable [3,4,5]) -- (Just 3) --- sumTwo doesn't handle when the list is empty or there's only one element in --- which case you get an error. -sumTwo [1] -- Failed pattern match +second :: forall a. List a -> Maybe a +second Nil = Nothing +second (_ : Nil) = Nothing +second (_ : (y : _)) = Just y +second (fromFoldable [3,4,5]) -- (Just 4) -- Complementing patterns to match -- Good ol' Fibonacci -let fib 1 = 1 - fib 2 = 2 - fib x = fib (x-1) + fib (x-2) +fib 1 = 1 +fib 2 = 2 +fib x = fib (x-1) + fib (x-2) fib 10 -- 89 -- Use underscore to match any, where you don't care about the binding name -let isZero 0 = true - isZero _ = false +isZero 0 = true +isZero _ = false +isZero 9 -- false -- Pattern matching on records -let ecoTitle {author = "Umberto Eco", title = t} = Just t - ecoTitle _ = Nothing +ecoTitle {author: "Umberto Eco", title: t} = Just t +ecoTitle _ = Nothing -ecoTitle book -- Just ("Foucault's pendulum") +ecoTitle {title: "Foucault's pendulum", author: "Umberto Eco"} -- (Just "Foucault's pendulum") ecoTitle {title: "The Quantum Thief", author: "Hannu Rajaniemi"} -- Nothing -- ecoTitle requires both field to type check: ecoTitle {title: "The Quantum Thief"} -- Object lacks required property "author" @@ -181,13 +179,13 @@ ecoTitle {title: "The Quantum Thief"} -- Object lacks required property "author" -- Lambda expressions (\x -> x*x) 3 -- 9 (\x y -> x*x + y*y) 4 5 -- 41 -let sqr = \x -> x*x +sqr = \x -> x*x -- Currying -let myAdd x y = x + y -- is equivalent with -let myAdd' = \x -> \y -> x + y -let add3 = myAdd 3 -:t add3 -- Prim.Int -> Prim.Int +myAdd x y = x + y -- is equivalent with +myAdd' = \x -> \y -> x + y +add3 = myAdd 3 +:t add3 -- Int -> Int -- Forward and backward function composition -- drop 3 followed by taking 5 @@ -196,7 +194,7 @@ let add3 = myAdd 3 (drop 3 <<< take 5) (1..20) -- [4,5] -- Operations using higher order functions -let even x = x `mod` 2 == 0 +even x = x `mod` 2 == 0 filter even (1..10) -- [2,4,6,8,10] map (\x -> x + 11) (1..5) -- [12,13,14,15,16]