Prelude API
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    Function filter

    • Creates a transformer that filters values based on a predicate function.

      Type Parameters

      • T

        The type of values in the input async iterable

      • U

        The narrowed type when using a type predicate (extends T)

      Parameters

      • predicate: TypePredicate<T, U>

        Function to test each value with its index

      Returns Transformer<T, U>

      A transformer function that yields only values that pass the predicate

      This function creates a transformer that only yields values from an async iterable that pass a predicate test. The predicate can be a synchronous function, an async function, or a type predicate for TypeScript type narrowing.

      The filter function supports three different predicate types:

      • Regular predicates that return a boolean
      • Async predicates that return a Promise
      • Type predicates that narrow the type (e.g., is User)
      // Filter even numbers
      const evenNumbers = await G.pipe(
        G.ofIterable([1, 2, 3, 4, 5, 6]),
        G.filter(x => x % 2 === 0),
        G.array
      ); // [2, 4, 6]

      // Async predicate
      const validUsers = await G.pipe(
        G.ofIterable(users),
        G.filter(async user => {
          const isValid = await validateUser(user);
          return isValid;
        }),
        G.array
      );

      // Type predicate (narrows the type)
      interface User { id: number; name: string; }
      interface Admin extends User { role: 'admin'; permissions: string[]; }

      function isAdmin(user: User): user is Admin {
        return 'role' in user && user.role === 'admin';
      }

      const admins = await G.pipe(
        G.ofIterable(users),
        G.filter(isAdmin), // Narrows type from User to Admin
        G.array
      );

    • Creates a transformer that filters values based on a predicate function.

      Type Parameters

      • T

        The type of values in the input async iterable

      Parameters

      Returns Transformer<T>

      A transformer function that yields only values that pass the predicate

      This function creates a transformer that only yields values from an async iterable that pass a predicate test. The predicate can be a synchronous function, an async function, or a type predicate for TypeScript type narrowing.

      The filter function supports three different predicate types:

      • Regular predicates that return a boolean
      • Async predicates that return a Promise
      • Type predicates that narrow the type (e.g., is User)
      // Filter even numbers
      const evenNumbers = await G.pipe(
        G.ofIterable([1, 2, 3, 4, 5, 6]),
        G.filter(x => x % 2 === 0),
        G.array
      ); // [2, 4, 6]

      // Async predicate
      const validUsers = await G.pipe(
        G.ofIterable(users),
        G.filter(async user => {
          const isValid = await validateUser(user);
          return isValid;
        }),
        G.array
      );

      // Type predicate (narrows the type)
      interface User { id: number; name: string; }
      interface Admin extends User { role: 'admin'; permissions: string[]; }

      function isAdmin(user: User): user is Admin {
        return 'role' in user && user.role === 'admin';
      }

      const admins = await G.pipe(
        G.ofIterable(users),
        G.filter(isAdmin), // Narrows type from User to Admin
        G.array
      );

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