Multiple values in single-value context

Question

Due to error handling in Go  I often end up with multiple values functions  So far  the way I have managed this has been very messy and I am looking for best practices to write cleaner code   Let s say I have the following function   type Item struct      Value int    Name string    func Get value int   Item  error         some code    return item  nil     How can I assign a new variable to item Value elegantly  Before introducing the error handling  my function just returned item and I could simply do this   val    Get 1  Value   Now I do this   item       Get 1  val    item Value   Isn t there a way to access directly the first returned variable

User · Answer

Here s a generic helper function with assumption checking  func assumeNoError value interface    err error  interface         if err    nil           panic  quot error encountered when none assumed  quot    err Error              return value    Since this returns as an interface    you ll generally need to cast it back to your function s return type  For example  the OP s example called Get 1   which returns  Item  error   item    assumeNoError Get 1    Item    The trick that makes this possible  Multi-values returned from one function call can be passed in as multi-variable arguments to another function   As a special case  if the return values of a function or method g are equal in number and individually assignable to the parameters of another function or method f  then the call f g parameters of g   will invoke f after binding the return values of g to the parameters of f in order    This answer borrows heavily from existing answers  but none had provided a simple  generic solution of this form

User · Answer

No  you cannot directly access the first value   I suppose a hack for this would be to return an array of values instead of  item  and  err   and then just do     item       Get 1  0  but I would not recommend this

User · Answer

No  but that is a good thing since you should always handle your errors   There are techniques that you can employ to defer error handling  see Errors are values by Rob Pike    ew     amp errWriter w  fd  ew write p0 a b   ew write p1 c d   ew write p2 e f      and so on if ew err    nil       return ew err      In this example from the blog post he illustrates how you could create an errWriter type that defers error handling till you are done calling write

User · Answer

How about this way   package main  import        fmt       errors     type Item struct       Value int     Name string    var items   Item     Item  Value 0  Name  zero                              Value 1  Name  one                              Value 2  Name  two     func main         var err error     v    Get 3   amp err  Value     if err    nil           fmt Println err          return           fmt Println v      func Get value int  err  error  Item       if value  gt   len items  - 1             err   errors New  error           return Item         else           return items value

User · Answer

In case of a multi-value return function you can t refer to fields or methods of a specific value of the result when calling the function   And if one of them is an error  it s there for a reason  which is the function might fail  and you should not bypass it because if you do  your subsequent code might also fail miserably  e g  resulting in runtime panic    However there might be situations where you know the code will not fail in any circumstances  In these cases you can provide a helper function  or method  which will discard the error  or raise a runtime panic if it still occurs   This can be the case if you provide the input values for a function from code  and you know they work  Great examples of this are the template and regexp packages  if you provide a valid template or regexp at compile time  you can be sure they can always be parsed without errors at runtime  For this reason the template package provides the Must t  Template  err error   Template function and the regexp package provides the MustCompile str string   Regexp function  they don t return errors because their intended use is where the input is guaranteed to be valid   Examples       text  is a valid template  parsing it will not fail var t   template Must template New  name   Parse  text           a-z     0-9       is a valid regexp  always compiles var validID   regexp MustCompile    a-z     0-9          Back to your case  IF you can be certain Get   will not produce error for certain input values  you can create a helper Must   function which would not return the error but raise a runtime panic if it still occurs   func Must i Item  err error  Item       if err    nil           panic err            return i     But you should not use this in all cases  just when you re sure it succeeds  Usage   val    Must Get 1   Value   Alternative   Simplification  You can even simplify it further if you incorporate the Get   call into your helper function  let s call it MustGet   func MustGet value int  Item       i  err    Get value      if err    nil           panic err            return i     Usage   val    MustGet 1  Value   See some interesting   related questions   how to parse multiple returns in golang  Return map like   39 ok  39  in Golang on normal functions

User · Answer

Yes  there is   Surprising  huh  You can get a specific value from a multiple return using a simple mute function   package main  import  fmt  import  strings   func    a    interface      interface         return a    type A struct       B string     C func   string     func main         a    A           B strings TrimSpace    E    1   string            C    G    0   func   string               fmt Printf    s says  s n   a B  a C       func E    bool  string        return false   F     func G    func   string   bool        return func   string   return  Hello     true     https   play golang org p IwqmoKwVm-  Notice how you select the value number just like you would from a slice array and then the type to get the actual value   You can read more about the science behind that from this article  Credits to the author

[go] Multiple values in single-value context

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