feat(stdlib): Add List.filterMap, List.filterMapi, List.findMap (#2201)

spotandjake · ospencer · web-flow · commit 551f5adf8f36 · 2025-01-02T20:39:46.000Z
Co-authored-by: Oscar Spencer &lt;oscar.spen@gmail.com&gt;
diff --git a/compiler/test/stdlib/list.test.gr b/compiler/test/stdlib/list.test.gr
@@ -88,6 +88,19 @@ assert filteri((x, i) => i + x > 2, list) == [2, 3]
 assert filteri((x, i) => x == 3, list) == [3]
 assert filteri((x, i) => x == 3, []) == []
 
+// List.filterMap
+
+assert List.filterMap(x => Some(x), list) == list
+assert List.filterMap(x => if (x == 3) Some(4) else None, list) == [4]
+assert List.filterMap(x => None, list) == []
+
+// List.filterMapi
+assert List.filterMapi((x, i) => Some(x), list) == list
+assert List.filterMapi((x, i) => if (x == 3) Some(4) else None, list) == [4]
+assert List.filterMapi((x, i) => None, list) == []
+assert List.filterMapi((x, i) => if (i != 0) Some(i) else None, list) == [1, 2]
+assert List.filterMapi((x, i) => if (i != 0) Some(x) else None, list) == [2, 3]
+
 // List.reject
 
 assert reject(x => x > 0, list) == []
@@ -207,6 +220,13 @@ assert findIndex(x => x == 1, list) == Some(0)
 assert findIndex(x => x == 2, list) == Some(1)
 assert findIndex(x => false, list) == None
 
+// List.findMap
+let duplicateList = [(1, 'a'), (2, 'b'), (1, 'c')]
+assert List.findMap(((k, v)) => if (k == 1) Some(v) else None, duplicateList) ==
+  Some('a')
+assert List.findMap(x => if (x == 2) Some(x) else None, list) == Some(2)
+assert List.findMap(x => None, list) == None
+
 // List.product
 
 let listA = [1, 2]
diff --git a/stdlib/list.gr b/stdlib/list.gr
@@ -217,6 +217,56 @@ provide let mapi = (fn, list) => {
   iter(fn, list, 0)
 }
 
+/**
+ * Produces a new list initialized with the results of a mapper function
+ * called on each element of the input list.
+ * The mapper function can return `None` to exclude the element from the new list.
+ *
+ * @param fn: The mapper function to call on each element, where the value returned will be used to initialize the element in the new list
+ * @param list: The list to iterate
+ * @returns The new list with filtered mapped values
+ *
+ * @example List.filterMap(x => if (x % 2 == 0) Some(toString(x)) else None, [1, 2, 3, 4]) == ["2", "4"]
+ *
+ * @since v0.7.0
+ */
+provide let rec filterMap = (fn, list) => {
+  match (list) {
+    [] => [],
+    [first, ...rest] => match (fn(first)) {
+      Some(v) => [v, ...filterMap(fn, rest)],
+      None => filterMap(fn, rest),
+    },
+  }
+}
+
+/**
+ * Produces a new list initialized with the results of a mapper function
+ * called on each element of the input list and its index.
+ * The mapper function can return `None` to exclude the element from the new list.
+ *
+ * @param fn: The mapper function to call on each element, where the value returned will be used to initialize the element in the new list
+ * @param list: The list to iterate
+ * @returns The new list with filtered mapped values
+ *
+ * @example List.filterMapi((x, i) => if (x % 2 == 0) Some(toString(x)) else None, [1, 2, 3, 4]) == ["2", "4"]
+ * @example List.filterMapi((x, i) => if (i == 0) Some(toString(x)) else None, [1, 2, 3, 4]) == ["1"]
+ *
+ * @since v0.7.0
+ */
+provide let filterMapi = (fn, list) => {
+  let rec iter = (fn, list, index) => {
+    match (list) {
+      [] => [],
+      [first, ...rest] => match (fn(first, index)) {
+        Some(v) => [v, ...iter(fn, rest, index + 1)],
+        None => iter(fn, rest, index + 1),
+      },
+    }
+  }
+  iter(fn, list, 0)
+}
+
 /**
  * Produces a new list by calling a function on each element
  * of the input list. Each iteration produces an intermediate
@@ -242,7 +292,7 @@ provide let rec flatMap = (fn, list) => {
  *
  * @param fn: The function to call on each element, where the returned value indicates if the element satisfies the condition
  * @param list: The list to check
- * @returns `true` if all elements satify the condition or `false` otherwise
+ * @returns `true` if all elements satisfy the condition or `false` otherwise
  *
  * @since v0.1.0
  */
@@ -260,7 +310,7 @@ provide let rec every = (fn, list) => {
  *
  * @param fn: The function to call on each element, where the returned value indicates if the element satisfies the condition
  * @param list: The list to iterate
- * @returns `true` if one or more elements satify the condition or `false` otherwise
+ * @returns `true` if one or more elements satisfy the condition or `false` otherwise
  *
  * @since v0.1.0
  */
@@ -769,7 +819,7 @@ provide let rec takeWhile = (fn, list) => {
 }
 
 /**
- * Finds the first element in a list that satifies the given condition.
+ * Finds the first element in a list that satisfies the given condition.
  *
  * @param fn: The function to call on each element, where the returned value indicates if the element satisfies the condition
  * @param list: The list to search
@@ -787,7 +837,7 @@ provide let rec find = (fn, list) => {
 }
 
 /**
- * Finds the first index in a list where the element satifies the given condition.
+ * Finds the first index in a list where the element satisfies the given condition.
  *
  * @param fn: The function to call on each element, where the returned value indicates if the element satisfies the condition
  * @param list: The list to search
@@ -808,6 +858,31 @@ provide let findIndex = (fn, list) => {
   findItemIndex(list, 0)
 }
 
+/**
+ * Finds the first element in a list that satisfies the given condition and
+ * returns the result of applying a mapper function to it.
+ *
+ * @param fn: The function to call on each element, where the returned value indicates if the element satisfies the condition
+ * @param list: The list to search
+ * @returns `Some(mapped)` containing the first value found with the given mapping or `None` otherwise
+ *
+ * @example
+ * let jsonObject = [(1, 'a'), (2, 'b'), (1, 'c')]
+ * let getItem = (key, obj) => List.findMap(((k, v)) => if (k == key) Some(v) else None, obj)
+ * assert getItem(1, jsonObject) == Some('a')
+ *
+ * @since v0.7.0
+ */
+provide let rec findMap = (fn, list) => {
+  match (list) {
+    [] => None,
+    [first, ...rest] => match (fn(first)) {
+      None => findMap(fn, rest),
+      Some(v) => Some(v),
+    },
+  }
+}
+
 /**
  * Combines two lists into a Cartesian product of tuples containing
  * all ordered pairs `(a, b)`.
diff --git a/stdlib/list.md b/stdlib/list.md
@@ -343,6 +343,78 @@ Returns:
 |----|-----------|
 |`List<b>`|The new list with mapped values|
 
+### List.**filterMap**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+filterMap : (fn: (a => Option<b>), list: List<a>) => List<b>
+```
+
+Produces a new list initialized with the results of a mapper function
+called on each element of the input list.
+The mapper function can return `None` to exclude the element from the new list.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`fn`|`a => Option<b>`|The mapper function to call on each element, where the value returned will be used to initialize the element in the new list|
+|`list`|`List<a>`|The list to iterate|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`List<b>`|The new list with filtered mapped values|
+
+Examples:
+
+```grain
+List.filterMap(x => if (x % 2 == 0) Some(toString(x)) else None, [1, 2, 3, 4]) == ["2", "4"]
+```
+
+### List.**filterMapi**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+filterMapi : (fn: ((a, Number) => Option<b>), list: List<a>) => List<b>
+```
+
+Produces a new list initialized with the results of a mapper function
+called on each element of the input list and its index.
+The mapper function can return `None` to exclude the element from the new list.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`fn`|`(a, Number) => Option<b>`|The mapper function to call on each element, where the value returned will be used to initialize the element in the new list|
+|`list`|`List<a>`|The list to iterate|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`List<b>`|The new list with filtered mapped values|
+
+Examples:
+
+```grain
+List.filterMapi((x, i) => if (x % 2 == 0) Some(toString(x)) else None, [1, 2, 3, 4]) == ["2", "4"]
+```
+
+```grain
+List.filterMapi((x, i) => if (i == 0) Some(toString(x)) else None, [1, 2, 3, 4]) == ["1"]
+```
+
 ### List.**flatMap**
 
 <details disabled>
@@ -397,7 +469,7 @@ Returns:
 
 |type|description|
 |----|-----------|
-|`Bool`|`true` if all elements satify the condition or `false` otherwise|
+|`Bool`|`true` if all elements satisfy the condition or `false` otherwise|
 
 ### List.**some**
 
@@ -424,7 +496,7 @@ Returns:
 
 |type|description|
 |----|-----------|
-|`Bool`|`true` if one or more elements satify the condition or `false` otherwise|
+|`Bool`|`true` if one or more elements satisfy the condition or `false` otherwise|
 
 ### List.**forEach**
 
@@ -1124,7 +1196,7 @@ Returns:
 find : (fn: (a => Bool), list: List<a>) => Option<a>
 ```
 
-Finds the first element in a list that satifies the given condition.
+Finds the first element in a list that satisfies the given condition.
 
 Parameters:
 
@@ -1158,7 +1230,7 @@ Returns:
 findIndex : (fn: (a => Bool), list: List<a>) => Option<Number>
 ```
 
-Finds the first index in a list where the element satifies the given condition.
+Finds the first index in a list where the element satisfies the given condition.
 
 Parameters:
 
@@ -1173,6 +1245,41 @@ Returns:
 |----|-----------|
 |`Option<Number>`|`Some(index)` containing the index of the first element found or `None` otherwise|
 
+### List.**findMap**
+
+<details disabled>
+<summary tabindex="-1">Added in <code>next</code></summary>
+No other changes yet.
+</details>
+
+```grain
+findMap : (fn: (a => Option<b>), list: List<a>) => Option<b>
+```
+
+Finds the first element in a list that satisfies the given condition and
+returns the result of applying a mapper function to it.
+
+Parameters:
+
+|param|type|description|
+|-----|----|-----------|
+|`fn`|`a => Option<b>`|The function to call on each element, where the returned value indicates if the element satisfies the condition|
+|`list`|`List<a>`|The list to search|
+
+Returns:
+
+|type|description|
+|----|-----------|
+|`Option<b>`|`Some(mapped)` containing the first value found with the given mapping or `None` otherwise|
+
+Examples:
+
+```grain
+let jsonObject = [(1, 'a'), (2, 'b'), (1, 'c')]
+let getItem = (key, obj) => List.findMap(((k, v)) => if (k == key) Some(v) else None, obj)
+assert getItem(1, jsonObject) == Some('a')
+```
+
 ### List.**product**
 
 <details disabled>
