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We also recommend that a 185 | file or class name and description of purpose be included on the 186 | same "printed page" as the copyright notice for easier 187 | identification within third-party archives. 188 | 189 | Copyright [yyyy] [name of copyright owner] 190 | 191 | Licensed under the Apache License, Version 2.0 (the "License"); 192 | you may not use this file except in compliance with the License. 193 | You may obtain a copy of the License at 194 | 195 | http://www.apache.org/licenses/LICENSE-2.0 196 | 197 | Unless required by applicable law or agreed to in writing, software 198 | distributed under the License is distributed on an "AS IS" BASIS, 199 | WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 200 | See the License for the specific language governing permissions and 201 | limitations under the License. 202 | -------------------------------------------------------------------------------- /README.md: -------------------------------------------------------------------------------- 1 | # type-systems-showdown 2 | A wide ranging comparison-by-example of the typesystem features in various FP languages. Initially Purescript and Typescript. 3 | 4 | The aim is compare the things that overlap. We don't want this comparison table to turn into this - 5 | 6 | ![Compare appliances](https://imgs.xkcd.com/comics/appliances_2x.png) 7 | 8 | ## Untagged unions and friends 9 | 10 | Purescript doesn't support untagged unions. But it can do something similar with a [library](https://github.com/jvliwanag/purescript-untagged-union). TODO: Comapre and contrast with Typescript. 11 | 12 | Typescript 13 | ```typescript 14 | type t1 = 1 | 2 | 3 15 | 16 | type t2 = string | number 17 | 18 | type t3 = t1 | t2 19 | ``` 20 | 21 | In all these cases there is no boxing. 22 | 23 | Refinement of the type is done with [Flow sensitive typing](https://en.wikipedia.org/wiki/Flow-sensitive_typing) 24 | 25 | E.g. - 26 | Typescript 27 | ```typescript 28 | // t :: t2 29 | if(typeof t == "number") { 30 | // Here the type of t is refined to number 31 | } 32 | ``` 33 | 34 | This can also work for user defined types with [User defined type guards](https://www.typescriptlang.org/docs/handbook/advanced-types.html#user-defined-type-guards). These however are completely unsafe and the compiler does not check if your type check is correct. This is effectively unsafeCoerce but localised to the type check function. 35 | 36 | e.g. - 37 | Typescript 38 | ```typescript 39 | function isFish(pet: Fish | Bird): pet is Fish { 40 | return (pet as Fish).swim !== undefined; 41 | } 42 | 43 | // Both calls to 'swim' and 'fly' are now okay. 44 | let pet = getSmallPet(); 45 | 46 | if (isFish(pet)) { 47 | pet.swim(); 48 | } else { 49 | pet.fly(); 50 | } 51 | ``` 52 | 53 | *Structural* type narrowing is possible with `in` operator to check for fields - 54 | 55 | Typescript - 56 | ```typescript 57 | function move(pet: Fish | Bird) { 58 | if ("swim" in pet) { 59 | return pet.swim(); 60 | } 61 | return pet.fly(); 62 | } 63 | ``` 64 | 65 | ## Tagged Unions 66 | 67 | Purescript 68 | ```purescript 69 | data Shape = Square Number | Rectangle Number Number | Circle Number Number 70 | area :: Shape -> Number 71 | area s = case s of 72 | Square size -> size * size 73 | Rectangle width height -> width * height 74 | Circle radius -> Math.PI * radius * radius 75 | ``` 76 | 77 | Typescript requires defining records with a literal discriminant property for each variant. 78 | ```typescript 79 | interface Square { 80 | kind: "square"; 81 | size: number; 82 | } 83 | 84 | interface Rectangle { 85 | kind: "rectangle"; 86 | width: number; 87 | height: number; 88 | } 89 | 90 | interface Circle { 91 | kind: "circle"; 92 | radius: number; 93 | } 94 | 95 | type Shape = Square | Rectangle | Circle; 96 | 97 | function area(s: Shape) { 98 | // In the following switch statement, the type of s is narrowed in each case clause 99 | // according to the value of the discriminant property, thus allowing the other properties 100 | // of that variant to be accessed without a type assertion. 101 | switch (s.kind) { 102 | case "square": return s.size * s.size; 103 | case "rectangle": return s.width * s.height; 104 | case "circle": return Math.PI * s.radius * s.radius; 105 | } 106 | } 107 | ``` 108 | 109 | Typescript's implementation is clearly clunkier, but it does allow discriminating on multiple properties, as well as discriminating on a subset of the variants. 110 | 111 | Typescript 112 | ```typescript 113 | interface FailState { 114 | error: "1"; 115 | msg: string; 116 | } 117 | 118 | interface LoadingState { 119 | error: "0"; 120 | } 121 | 122 | interface SuccessState { 123 | error: "0"; 124 | value: number; 125 | } 126 | 127 | type State = FailState | LoadingState | SuccessState; 128 | 129 | function processState(state : State) { 130 | switch (state.error) { 131 | case "0": { 132 | console.log("We did not fail!"); 133 | break; 134 | } 135 | case "1": { 136 | console.log("We failed with ", state.msg); 137 | break; 138 | } 139 | } 140 | } 141 | 142 | processState({ msg: "unknown error", error: "1" }); 143 | ``` 144 | 145 | ## Variants 146 | 147 | Typescript supports Variants natively with untagged unions as shown in the previous examples. 148 | Purescript can have variants with the [Purescript-variant](https://github.com/natefaubion/purescript-variant) library. 149 | 150 | **TODO: Elaborate.** 151 | 152 | Challenge - declare a type which is similar to a previously defined type, but excludes a variant branch. 153 | 154 | More specifically, given a type - 155 | 156 | ```purescript 157 | data T = A | B | C Int 158 | ``` 159 | 160 | Now define a type function to generate a type U which is equivalent to a type - 161 | 162 | ```purescript 163 | data U = B | C Int 164 | ``` 165 | 166 | In Typescript you can define the second type in terms of the first like this - 167 | ```typescript 168 | type U = Exclude 169 | const f = (t: T): U => t === 'A' ? 'B' : t 170 | ``` 171 | 172 | [Try it online!](https://www.typescriptlang.org/play?#code/C4TwDgpgBAKlC8UDkBBJUA+yBC6sG0kBhJAGigDsBXAWwCMIAnAXQChXRIoBVBKAUQAeAYwA2VACYQAPDHKokAPnbCA9hQDOwKADM+ACmAAuWAEoTveIqjb4d5GigB+HOhPAgA) 173 | 174 | In Purescript we can denote the general relationship between any two data variants as long as the first has the field `A` and the second has the corresponding field `B` - 175 | ```purescript 176 | _A = (SProxy :: _ “A”) 177 | _B = (SProxy :: _ “B”) 178 | f :: forall r. Variant (A::String | r) -> Variant (B::String | r) 179 | f = on _A (inj _B) identity 180 | ``` 181 | 182 | ## Records 183 | 184 | Homogenous records Challenge! - Write a function signature where the input is some arbitrary record and the output is a record with the same fields but all types converted to Number - 185 | 186 | Typescript version uses [Index types](https://www.typescriptlang.org/docs/handbook/advanced-types.html#index-types) 187 | ```typescript 188 | const f = (_: {[K in keyof O]: number}) => ... 189 | ``` 190 | 191 | [Try it online!](https://www.typescriptlang.org/play?#code/MYewdgzgLgBAhgLhgbzHAtgUydATgSzAHMAaeI7GMAV3QCNNcBfGAXhTSyQHIAzEENzJwKSAEwAGJgG4AULNCRYqDJQD6ZAHTbcmaC3ZwF4aDF5sYAHgDyMTAA8omMABMIMamADWYEAHcwAD4ACjUkZABtAGkYQhgvTABPEHNrAF0kGnpGJgBKNkCUJlleYN1oXKA) 192 | 193 | Challenge: Represent a record that does *NOT* have a particular label - 194 | 195 | Typescript version uses `never`. The following function will not accept a record with field `foo`. 196 | ```typescript 197 | const f = (_: Record & {foo?: never}) => {} 198 | ``` 199 | 200 | [Try it online!](https://www.typescriptlang.org/play?#code/MYewdgzgLgBAZjAvDAFAfQFwwEoFNQBOAJgDzQECWYA5gDQxgCuAtgEa4EB8MAZDAN5wQIAPxYwuAG4cAvgEok3fjIBQKuCmVz1mgIZYAjPJ2Dhh+UA) 201 | 202 | Challenge: Represent a record which is the union of two arbitrary records - 203 | 204 | Typescript again uses index types to great effect! The following function will accept three records where the third must necessarily be a union of the first two - 205 | ```typescript 206 | const f = (a: A, b: {[K in keyof A]: A[K] | null}) => {} 207 | ``` 208 | 209 | [Try it online!](https://www.typescriptlang.org/play?#code/MYewdgzgLgBAZjAvDAPAQRgUwB5U2AEwhgFcwBrMEAdzAD4AKAQwC4Y0AaGAIzYG8A2gGkYASzAxymAJ4gEaALps0whTAA+MMCQA2OgL4BKJHRh99AKAtwGfOCBBsAjF25MATmwDk0d+IDmXvpcdg5s2nquHt5u7kGG1rb2jjAuPNEwPlB+YIHBZsneETpeUZ6ZsfGJoSlpsd6+AUEhhVq6Bgk2Nc5lDdlN+Xz1FR7xQA) 210 | 211 | 212 | ## Dynamic record manipulation 213 | 214 | Challenge: Can we add fields dynamically, and have them be tracked? For example, can you write a function `addField :: Record -> String -> a -> Record` so that the output record is the input record with a field added? 215 | 216 | Typescript can do it but by using records instead of fieldnames. 217 | 218 | So this doesn't work - 219 | ```typescript 220 | const add = (o: O, k: K, v: V): O & {[K]: V} => ({...o, [k]: v}) 221 | const set = (o: O, k: K, v: O[K]): O => ({...o, [k]: v}) 222 | const res = set(add({a:1, c:'as'}, 'b', 3), 'b', 33) 223 | console.log(res) 224 | ``` 225 | 226 | But this does - 227 | ```typescript 228 | const add = (a: A, b: B) => ({...a, ...b}) 229 | const set = (o: O, k: K, v: O[K]): O => ({...o, [k]: v}) 230 | const res = set(add({a:1, c:'as'}, {b: 3}), 'b', 33) 231 | const res2 = set(add({a:1, c:'as'}, {b: 3}), 'c', 33) // fails 232 | const res3 = set(add({a:1, c:'as'}, {b: 3}), 'c', 'string works') 233 | const res4 = set(add({a:1, c:'as'}, {b: 3}), 'd', 'string works') // fails 234 | ``` 235 | 236 | ## Dynamic Record fieldname manipulation 237 | 238 | One of the newer shiny things in typescript is `Uppercase`, `Capitalise` etc. 239 | 240 | E.g. - 241 | ```typescript 242 | type A = Uppercase<'foo'> // Is the same as type A = 'FOO' 243 | ``` 244 | 245 | Also you can do - 246 | ```typescript 247 | type Getter = `get${Capitalize} ` 248 | type B = Getter<'fooBar'> // Becomes 'getFooBar' 249 | 250 | // And use `never` to disallow 251 | type FromGetter = A extends get${infer B} ? Uncapitalize : never 252 | type C = FromGetter<'willFail'> 253 | type D = FromGetter<'getThisWillWork'>; 254 | ``` 255 | 256 | Dynamic field manipulation is possible in Purescript as well but clunkier. See for example https://qiita.com/kimagure/items/4f5c6054870f631ff768. 257 | --------------------------------------------------------------------------------