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72 lines
2.4 KiB
Go
72 lines
2.4 KiB
Go
// Copyright (c) 2023 - 2025 IBM Corp.
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// All rights reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package monoid
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import (
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F "github.com/IBM/fp-go/v2/function"
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S "github.com/IBM/fp-go/v2/semigroup"
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)
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// FunctionMonoid creates a monoid for functions when the codomain (return type) has a monoid.
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//
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// Given a monoid for type B, this creates a monoid for functions of type func(A) B.
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// The resulting monoid combines functions by combining their results using the provided
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// monoid's Concat operation. The empty function always returns the monoid's Empty value.
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//
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// This allows you to compose functions that return monoidal values in a point-wise manner.
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//
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// Type Parameters:
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// - A: The domain (input type) of the functions
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// - B: The codomain (return type) of the functions, which must have a monoid
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//
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// Parameters:
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// - m: A monoid for the codomain type B
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//
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// Returns:
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// - A Monoid[func(A) B] that combines functions point-wise
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//
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// Example:
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//
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// // Monoid for functions returning integers
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// intAddMonoid := MakeMonoid(
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// func(a, b int) int { return a + b },
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// 0,
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// )
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//
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// funcMonoid := FunctionMonoid[string, int](intAddMonoid)
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//
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// // Define some functions
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// f1 := func(s string) int { return len(s) }
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// f2 := func(s string) int { return len(s) * 2 }
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//
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// // Combine functions: result(x) = f1(x) + f2(x)
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// combined := funcMonoid.Concat(f1, f2)
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// result := combined("hello") // len("hello") + len("hello")*2 = 5 + 10 = 15
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//
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// // Empty function always returns 0
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// emptyFunc := funcMonoid.Empty()
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// result := emptyFunc("anything") // 0
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//
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// // Verify identity laws
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// assert.Equal(t, f1("test"), funcMonoid.Concat(funcMonoid.Empty(), f1)("test"))
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// assert.Equal(t, f1("test"), funcMonoid.Concat(f1, funcMonoid.Empty())("test"))
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func FunctionMonoid[A, B any](m Monoid[B]) Monoid[func(A) B] {
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return MakeMonoid(
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S.FunctionSemigroup[A](m).Concat,
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F.Constant1[A](m.Empty()),
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)
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}
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