index2.go

     1  // run
     2  
     3  // Copyright 2021 The Go Authors. All rights reserved.
     4  // Use of this source code is governed by a BSD-style
     5  // license that can be found in the LICENSE file.
     6  
     7  // Testing various generic uses of indexing, both for reads and writes.
     8  
     9  package main
    10  
    11  import "fmt"
    12  
    13  // Can index an argument (read/write) constrained to be a slice or an array.
    14  func Index1[T interface{ []int64 | [5]int64 }](x T) int64 {
    15  	x[2] = 5
    16  	return x[3]
    17  }
    18  
    19  // Can index an argument (read) constrained to be a byte array or a string.
    20  func Index2[T interface{ []byte | string }](x T) byte {
    21  	return x[3]
    22  }
    23  
    24  // Can index an argument (write) constrained to be a byte array, but not a string.
    25  func Index2a[T interface{ []byte }](x T) byte {
    26  	x[2] = 'b'
    27  	return x[3]
    28  }
    29  
    30  // Can index an argument (read/write) constrained to be a map. Maps can't
    31  // be combined with any other type for indexing purposes.
    32  func Index3[T interface{ map[int]int64 }](x T) int64 {
    33  	x[2] = 43
    34  	return x[3]
    35  }
    36  
    37  // But the type of the map keys or values can be parameterized.
    38  func Index4[T any](x map[int]T) T {
    39  	var zero T
    40  	x[2] = zero
    41  	return x[3]
    42  }
    43  
    44  func test[T comparable](got, want T) {
    45  	if got != want {
    46  		panic(fmt.Sprintf("got %v, want %v", got, want))
    47  	}
    48  }
    49  
    50  func main() {
    51  	x := make([]int64, 4)
    52  	x[3] = 2
    53  	y := [5]int64{1, 2, 3, 4, 5}
    54  	z := "abcd"
    55  	w := make([]byte, 4)
    56  	w[3] = 5
    57  	v := make(map[int]int64)
    58  	v[3] = 18
    59  
    60  	test(Index1(x), int64(2))
    61  	test(Index1(y), int64(4))
    62  	test(Index2(z), byte(100))
    63  	test(Index2(w), byte(5))
    64  	test(Index2a(w), byte(5))
    65  	test(Index3(v), int64(18))
    66  	test(Index4(v), int64(18))
    67  }
    68
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