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Ruby - Default Args || vs Fetch vs Merge

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很常時候，需要給變數一個預設值，因此有以下方法，都可以設定預設值

||

只要是空值，或是 false, nil 就會回傳後面的預設值

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h = { "a" => 100, "b" => false , 'c' => nil}
#=> {"a"=>100, "b"=>false, "c"=>nil}
h['a'] || 8
#=> 100
h['b'] || 8
#=> 8
h['c'] || 8
#=> 8
h['d'] || 8
#=> 8

fetch

即使是 nil, false 也會回傳，只有在空值的時候回傳預設值

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h = { "a" => 100, "b" => false , 'c' => nil}
#=> {"a"=>100, "b"=>false, "c"=>nil}
h.fetch('a', 8)
#=> 100
h.fetch('b', 8)
#=> false
h.fetch('c', 8)
#=> nil
h.fetch('d', 8)
#=> 8

在處理應該存在的哈希鍵時，使用 fetch。

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heroes = { batman: 'Bruce Wayne', superman: 'Clark Kent' }
# bad - if we make a mistake we might not spot it right away
heroes[:batman] # => "Bruce Wayne"
heroes[:supermann] # => nil

# good - fetch raises a KeyError making the problem obvious
heroes.fetch(:supermann)

在使用 fetch 時，使用第二個參數設置默認值而不是使用自定義的邏輯。

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batman = { name: 'Bruce Wayne', is_evil: false }

# bad - if we just use || operator with falsy value we won't get the expected result
batman[:is_evil] || true # => true

# good - fetch work correctly with falsy values
batman.fetch(:is_evil, true) # => false

盡量用 fetch 加區塊而不是直接設定默認值。

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batman = { name: 'Bruce Wayne' }

# bad - if we use the default value, we eager evaluate it
# so it can slow the program down if done multiple times
batman.fetch(:powers, get_batman_powers) # get_batman_powers is an expensive call

# good - blocks are lazy evaluated, so only triggered in case of KeyError exception
batman.fetch(:powers) { get_batman_powers }

merge

只有在 merge 的參數裡有同樣的值，才會覆蓋掉 default 的值

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default = { "a" => 100, "b" => false , 'c' => nil }
args    = { "a" => 8 }

default.merge(args)
#=> {"a"=>8, "b"=>false, "c"=>nil}

官方文件： fetch merge

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