了解在Ruby中与lambda一起使用的注入行为 [英] Understanding the behaviour of inject used with a lambda in Ruby

问题描述

我经常将预先配置的lambda插入到"map"，"select"等可枚举的方法中. 但是注入"的行为似乎有所不同. 例如与

I often plug pre-configured lambdas into enumerable methods like 'map', 'select' etc. but the behavior of 'inject' seems to be different. e.g. with

mult4 = lambda {|item| item * 4 }

然后

(5..10).map &mult4

给我

[20, 24, 28, 32, 36, 40]

但是，如果我制作一个2参数的lambda以便像这样进行注入，

However, if I make a 2-parameter lambda for use with an inject like so,

multL = lambda {|product, n| product * n }

我想说

(5..10).inject(2) &multL

因为注入"的初始值具有可选的单个参数， 但这给了我...

since 'inject' has an optional single parameter for the initial value, but that gives me ...

irb(main):027:0> (5..10).inject(2) &multL
LocalJumpError: no block given
        from (irb):27:in `inject'
        from (irb):27

但是，如果我将'& multL'放入要插入的 second 参数中，那么它将起作用.

However, if I stuff the '&multL' into a second parameter to inject, then it works.

irb(main):028:0> (5..10).inject(2, &multL)
=> 302400

我的问题是为什么这样做有效，而不是先前的尝试?"

My question is "why does that work and not the previous attempt?"

推荐答案

那是

(5..10).map &mult4

工作和

(5..10).inject(2) &multL

不是在第一种情况下，ruby parens是隐式的，所以确实意味着

doesn't is that ruby parens are implicit in the first case, so it really means

(5..10).map(&mult4)

如果需要，可以在第二种情况下使用

if you wanted, for the second case you could use

(5..10).inject 2, &multL

parens技巧之外的方法仅适用于将块传递给方法，而不适用于lambda对象.

The outside the parens trick only works for passing blocks to a method, not lambda objects.

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