smalltalk block - can I explicitly set the returning value and stop executing the block?

问题

The return value of #value: message, when sent to a block, is the value of the last sentence in that block. So [ 1 + 2. 3 + 4. ] value evaluates to 7. I find that hard to use sometimes. Is there a way to explicitly set the returning value and stop executing the block?

For exercise, try rewriting this block without using my imaginary #return: message and see how ugly it gets. I must be missing something.

[ :one :two |
  one isNil ifTrue: [ two isNil ifTrue: [ self return: nil ] ifFalse: [ self return: true ] ].
  two ifNil: [ self return: false ].

 (one > two)
  ifTrue: [ self return: true ]
  ifFalse: [ (one < two)
              ifTrue: [ self return: false ]
              ifFalse: [ self return: nil ]
            ].
]

EDIT: self return: sth really is nonsense, but it does make sense at some level :)

回答1:

There's nothing like a guard clause - blah ifTrue: [^ foo] - inside a block, because ^ is a non-local return, returning from the method calling the block rather than the block itself.

Big blocks - like big anythings - should be refactored into smaller, more understandable/tractable subparts, but sometimes that's not always possible. I mean this answer to suggest options to try when you can't really simplify in the usual ways.

If your block is really that complicated, and you can't get it simpler (splitting it up delocalises the information too much, for instance) then perhaps you can use an explicit return value. In particular, if your block doesn't return nil you could do something like

[:one :two | | result |
    result := (one isNil and: [two isNil]) ifTrue: [false].
    result ifNil: ["do one thing, possibly setting result"].
    result]

If your block can return nil, you'll need another sentinel value:

[:one :two | | result marker |
    result := marker := Object new.
    (result == marker) ifTrue: ["do one thing, possibly setting result"].
    result]

Lastly - and I hesitate to suggest this - you could do this:

[1 + 2.
thisContext return: 5.
3 + 4] value

which returns 5.

(Verifying how this interacts with ^ and inlined selectors like #ifTrue:ifFalse: left as an exercise for the reader.)

回答2:

It seems that your code tries to handles nil like an infinity value when comparing one and two. The following code may be more readable depending on the context:

a := [:one :two |
    | x y |
    x := one ifNil: [Float infinity].
    y := two ifNil: [Float infinity].
    (x = y) ifTrue: [nil] ifFalse: [x > y]]

A useful feature of #ifTrue:ifFalse:, #ifNil:ifNotNil: and similar testing methods is that they return the value of the block that gets evaluated. e.g. (4 > 1) ifTrue: ['greater'] ifFalse: ['not-greater'] evaluates to 'greater'. This feature often makes it possible to return a value from a nested block in tail position.

When the code inside a block gets too complicated I suggest your refactor it to a method. But see Frank's answer for workarounds.

Edit:

As pointed out in the comments the code above assumes numbers. I also came up with something that works with other comparable objects:

a:=
[ :one :two |
  true caseOf: {
    [one = two]->[nil].
    [one isNil]->[true].
    [two isNil]->[false]
  } otherwise: [one>two]]

That #caseOf: construct is rarely used but it's certainly better than thisContext return:

回答3:

You'd like to implement some break, continue, exit... The usual way to control flow in Smalltalk is with blocks. So one funny solution is to use a helper method with a Block return value to break the flow, like described here .

Object>>exitThru: aBlock
    ^aBlock value: [:result | ^result]

Now, let see how to use it:

| aBlock |
aBlock :=   [ :one :two |
    self exitThru: [:exit |
        one isNil ifTrue: [ two isNil ifTrue: [exit value: nil ] ifFalse: [ exit value: true ] ].
        two isNil ifTrue: [ exit value: false ].
        one > two ifTrue: [ exit value: true ].
        one < two ifTrue: [ exit value: false ].
        exit value: nil] ].

#(('abc' nil) (nil nil) (nil 'def') ('y' 'abc') ('y' 'y') ('y' 'z'))
    collect:
        [:pair |
        aBlock value: pair first value: pair last ]
-> #(false nil true true nil false)

EDIT my first version was unnecessarily complex, can't remember what lead me to an additional indirection:

| aBlock |
aBlock :=  [:wrapOne :wrapTwo |
    self exitThru: [:exit |
        [ :one :two |
        one isNil ifTrue: [ two isNil ifTrue: [exit value: nil ] ifFalse: [ exit value: true ] ].
        two isNil ifTrue: [ exit value: false ].
        one > two ifTrue: [ exit value: true ].
        one < two ifTrue: [ exit value: false ].
        exit value: nil ]
            value: wrapOne value: wrapTwo ] ].

Well, more funny than usefull, I hope you will find more simple and expressive way to code.

来源：https://stackoverflow.com/questions/7547750/smalltalk-block-can-i-explicitly-set-the-returning-value-and-stop-executing-th

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