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proposal: Go 2: Lightweight anonymous function syntax #21498

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neild opened this issue Aug 17, 2017 · 40 comments

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@neild
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commented Aug 17, 2017

Many languages provide a lightweight syntax for specifying anonymous functions, in which the function type is derived from the surrounding context.

Consider a slightly contrived example from the Go tour (https://tour.golang.org/moretypes/24):

func compute(fn func(float64, float64) float64) float64 {
	return fn(3, 4)
}

var _ = compute(func(a, b float64) float64 { return a + b })

Many languages permit eliding the parameter and return types of the anonymous function in this case, since they may be derived from the context. For example:

// Scala
compute((x: Double, y: Double) => x + y)
compute((x, y) => x + y) // Parameter types elided.
compute(_ + _) // Or even shorter.
// Rust
compute(|x: f64, y: f64| -> f64 { x + y })
compute(|x, y| { x + y }) // Parameter and return types elided.

I propose considering adding such a form to Go 2. I am not proposing any specific syntax. In terms of the language specification, this may be thought of as a form of untyped function literal that is assignable to any compatible variable of function type. Literals of this form would have no default type and could not be used on the right hand side of a := in the same way that x := nil is an error.

Uses 1: Cap'n Proto

Remote calls using Cap'n Proto take an function parameter which is passed a request message to populate. From https://github.com/capnproto/go-capnproto2/wiki/Getting-Started:

s.Write(ctx, func(p hashes.Hash_write_Params) error {
  err := p.SetData([]byte("Hello, "))
  return err
})

Using the Rust syntax (just as an example):

s.Write(ctx, |p| {
  err := p.SetData([]byte("Hello, "))
  return err
})

Uses 2: errgroup

The errgroup package (http://godoc.org/golang.org/x/sync/errgroup) manages a group of goroutines:

g.Go(func() error {
  // perform work
  return nil
})

Using the Scala syntax:

g.Go(() => {
  // perform work
  return nil
})

(Since the function signature is quite small in this case, this might arguably be a case where the lightweight syntax is less clear.)

@griesemer

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commented Aug 17, 2017

I'm sympathetic to the general idea, but I find the specific examples given not very convincing: The relatively small savings in terms of syntax doesn't seem worth the trouble. But perhaps there are better examples or more convincing notation.

(Perhaps with the exception of the binary operator example, but I'm not sure how common that case is in typical Go code.)

@davecheney

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commented Aug 17, 2017

@ianlancetaylor ianlancetaylor changed the title Go 2: Lightweight anonymous function syntax proposal: Go 2: Lightweight anonymous function syntax Aug 17, 2017

@gopherbot gopherbot added this to the Proposal milestone Aug 17, 2017

@ianlancetaylor

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commented Aug 17, 2017

I think this is more convincing if we restrict its use to cases where the function body is a simple expression. If we are required to write a block and an explicit return, the benefits are somewhat lost.

Your examples then become

s.Write(ctx, p => p.SetData([]byte("Hello, "))

g.Go(=> nil)

The syntax is something like

[ Identifier ] | "(" IdentifierList ")" "=>" ExpressionList

This may only be used in an assignment to a value of function type (including assignment to a parameter in the process of a function call). The number of identifiers must match the number of parameters of the function type, and the function type determines the identifier types. The function type must have zero results, or the number of result parameters must match the number of expressions in the list. The type of each expression must be assignable to the type of the corresponding result parameter. This is equivalent to a function literal in the obvious way.

There is probably a parsing ambiguity here. It would also be interesting to consider the syntax

λ [Identifier] | "(" IdentifierList ")" "." ExpressionList

as in

s.Write(ctx, λp.p.SetData([]byte("Hello, "))
@neild

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commented Aug 17, 2017

A few more cases where closures are commonly used.

(I'm mainly trying to collect use cases at the moment to provide evidence for/against the utility of this feature.)

@faiface

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commented Aug 18, 2017

I actually like that Go doesn't discriminate longer anonymous functions, as Java does.

In Java, a short anonymous function, a lambda, is nice and short, while a longer one is verbose and ugly compared to the short one. I've even seen a talk/post somewhere (I can't find it now) that encouraged only using one-line lambdas in Java, because those have all those non-verbosity advantages.

In Go, we don't have this problem, both short and longer anonymous functions are relatively (but not too much) verbose, so there is no mental obstacle to using longer ones too, which is sometimes very useful.

@jimmyfrasche

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commented Aug 18, 2017

The shorthand is natural in functional languages because everything is an expression and the result of a function is the last expression in the function's definition.

Having a shorthand is nice so other languages where the above doesn't hold have adopted it.

But in my experience it's never as nice when it hits the reality of a language with statements.

It's either nearly as verbose because you need blocks and returns or it can only contain expressions so it's basically useless for all but the simplest of things.

Anonymous functions in Go are about as close as they can get to optimal. I don't see the value in shaving it down any further.

@bcmills

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commented Aug 24, 2017

It's not the func syntax that is the problem, it's the redundant type declarations.

Simply allowing the function literals to elide unambiguous types would go a long way. To use the Cap'n'Proto example:

s.Write(ctx, func(p) error { return p.SetData([]byte("Hello, ")) })
@neild

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commented Aug 24, 2017

Yes, it's the type declarations that really add noise. Unfortunately, "func (p) error" already has a meaning. Perhaps permitting _ to substitute in for an inferenced type would work?

s.Write(ctx, func(p _) _ { return p.SetData([]byte("Hello, ")) })

I rather like that; no syntactic change at all required.

@martisch

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commented Aug 25, 2017

I do not like the stutter of _. Maybe func could be replaced by a keyword that infers the type parameters:
s.Write(ctx, λ(p) { return p.SetData([]byte("Hello, ")) })

@davecheney

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commented Aug 25, 2017

Is this actually a proposal or are you just spitballing what Go would look like if you dressed it like Scheme for Halloween? I think this proposal is both unnecessary and in poor keeping with the language's focus on readability.

Please stop trying to change the syntax of the language just because it looks different to other languages.

@cespare

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commented Aug 25, 2017

I think that having a concise anonymous function syntax is more compelling in other languages that rely more on callback-based APIs. In Go, I'm not sure the new syntax would really pay for itself. It's not that there aren't plenty of examples where folks use anonymous functions, but at least in the code I read and write the frequency is fairly low.

@bcmills

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commented Aug 25, 2017

I think that having a concise anonymous function syntax is more compelling in other languages that rely more on callback-based APIs.

To some extent, that is a self-reinforcing condition: if it were easier to write concise functions in Go, we may well see more functional-style APIs. (Whether that is a good thing or not, I do not know.)

I do want to emphasize that there is a difference between "functional" and "callback" APIs: when I hear "callback" I think "asynchronous callback", which leads to a sort of spaghetti code that we've been fortunate to avoid in Go. Synchronous APIs (such as filepath.Walk or strings.TrimFunc) are probably the use-case we should have in mind, since those mesh better with the synchronous style of Go programs in general.

@dimitropoulos

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commented Oct 31, 2017

I would just like to chime in here and offer a use case where I have come to appreciate the arrow style lambda syntax to greatly reduces friction: currying.

consider:

// current syntax
func add(a int) func(int) int {
	return func(b int) int {
		return a + b
	}
}

// arrow version (draft syntax, of course)
add := (a int) => (b int) => a + b

func main() {
	add2 := add(2)
	add3 := add(3)
	fmt.Println(add2(5), add3(6))
}

Now imagine we are trying to curry a value into a mongo.FieldConvertFunc or something which requires a functional approach, and you'll see that having a more lightweight syntax can improve things quite a bit when switching a function from not being curried to being curried (happy to provide a more real-world example if anyone wants).

Not convinced? Didn't think so. I love go's simplicity too and think it's worth protecting.

Another situation that happens to me a lot is where you have and you want to now curry the next argument with currying.

now you would have to change
func (a, b) x
to
func (a) func(b) x { return func (b) { return ...... x } }

If there was an arrow syntax you would simply change
(a, b) => x
to
(a) => (b) => x

@myitcv

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commented Nov 6, 2017

@neild whilst I haven't contributed to this thread yet, I do have another use case that would benefit from something similar to what you proposed.

But this comment is actually about another way of dealing with the verbosity in calling code: have a tool like gocode (or similar) template a function value for you.

Taking your example:

func compute(fn func(float64, float64) float64) float64 {
	return fn(3, 4)
}

If we assume we had typed:

var _ = compute(
                ^

with the cursor at the position shown by the ^; then invoking such a tool could trivially template a function value for you giving:

var _ = compute(func(a, b float64) float64 { })
                                            ^

That would certainly cover the use case I had in mind; does it cover yours?

@neild

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commented Nov 6, 2017

Code is read much more often than it is written. I don't believe saving a little typing is worth a change to the language syntax here. The advantage, if there is one, would largely be in making code more readable. Editor support won't help with that.

A question, of course, is whether removing the full type information from an anonymous function helps or harms readability.

@mrkaspa

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commented Nov 20, 2017

I don't think this kind of syntax reduces readability, almost all modern programming languages have a syntax for this and thats because it encourages the use of functional style to reduce the boilerplate and make the code clearer and easier to maintain. It's a great pain to use anonymous functions in golang when they are passed as parameters to functions because you have to repeat yourself typing again the types that you know you must pass.

@hooluupog

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commented Nov 20, 2017

I support the proposal. It saves typing and helps readability.My use case,

// Type definitions and functions implementation.
type intSlice []int
func (is intSlice) Filter(f func(int) bool) intSlice { ... }
func (is intSlice) Map(f func(int) int) intSlice { ... }
func (is intSlice) Reduce(f func(int, int) int) int { ...  }
list := []int{...} 
is := intSlice(list)

without lightweight anonymous function syntax:

res := is.Map(func(i int)int{return i+1}).Filter(func(i int) bool { return i % 2 == 0 }).
             Reduce(func(a, b int) int { return a + b })

with lightweight anonymous function syntax:

res := is.Map((i) => i+1).Filter((i)=>i % 2 == 0).Reduce((a,b)=>a+b)
@firelizzard18

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commented Dec 25, 2017

The lack of concise anonymous function expressions makes Go less readable and violates the DRY principle. I would like to write and use functional/callback APIs, but using such APIs is obnoxiously verbose, as every API call must either use an already defined function or an anonymous function expression that repeats type information that should be quite clear from the context (if the API is designed correctly).

My desire for this proposal is not even remotely that I think Go should look or be like other languages. My desire is entirely driven by my dislike for repeating myself and including unnecessary syntactic noise.

@griesemer

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commented Jan 3, 2018

In Go, the syntax for function declarations deviates a bit from the regular pattern that we have for other declarations. For constants, types, variables we always have:

keyword name type value

For example:

const   c    int  = 0
type    t    foo
var     v    bool = true

In general, the type can be a literal type, or it can be a name. For functions this breaks down, the type always must be a literal signature. One could image something like:

type BinaryOp func(x, y Value) Value

func f BinaryOp { ... }

where the function type is given as a name. Expanding a bit, a BinaryOp closure could then perhaps be written as

BinaryOp{ return x.Add(y) }

which might go a long way to shorter closure notation. For instance:

vector.Apply(BinaryOp{ return x.Add(y) })

The main disadvantage is that parameter names are not declared with the function. Using the function type brings them "in scope", similar to how using a struct value x of type S brings a field f into scope in a selector expression x.f or a struct literal S{f: "foo"}.

Also, this requires an explicitly declared function type, which may only make sense if that type is very common.

Just another perspective for this discussion.

@dimitropoulos

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commented Jan 3, 2018

Readability comes first, that seems to be something we can all agree on.

But that said, one thing I want to also chime in on (since it doesn't look like anyone else said it explicitly) is that the question of readability is always going to hinge on what you're used to. Having a discussion as we are about whether it hurts or harms readability isn't going to get anywhere in my opinion.

@griesemer perhaps some perspective from your time working on V8 would be useful here. I (at least) can say I was very much happy with javascript's prior syntax for functions (function(x) { return x; }) which was (in a way) even heavier to read than Go's is right now. I was in @douglascrockford's "this new syntax is a waste of time" camp.

But, all the same, the arrow syntax happened and I accepted it because I had to. Today, though, having used it a lot more and gotten more comfortable with it, I can say that it helps readability tremendously. I used the case of currying (and @hooluupog brought up a similar case of "dot-chaining") where a lightweight syntax produces code that is lightweight without being overly clever.

Now when I see code that does things like x => y => z => ... and it is much easier to understand at a glance (again... because I'm familiar with it. not all that long ago I felt quite the opposite).

What I'm saying is: this discussion boils down to:

  1. When you aren't used to it, it seems really strange and borderline useless if not harmful to readability. Some people just have or don't have a feeling one way or another on this.
  2. The more functional programming you're doing, the more the need for such a syntax pronounces itself. I would guess that this has something to do with functional concepts (like partial application and currying) that introduce a lot of functions for tiny jobs which translates to noise for the reader.

The best thing we can do is provide more use-cases.

@firelizzard18

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commented Jan 3, 2018

In response to @dimitropoulos's comment, here's a rough summary of my view:

I want to use design patterns (such as functional programming) that would greatly benefit from this proposal, as their use with the current syntax is excessively verbose.

@griesemer

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commented Jan 3, 2018

@dimitropoulos I've been working on V8 alright, but that was building the virtual machine, which was written in C++. My experience with actual Javascript is limited. That said, Javascript is a dynamically typed language, and without types much of the typing goes away. As several people have brought up before, a major issue here is the need to repeat types, a problem that doesn't exist in Javascript.

Also, for the record: In the early days of designing Go we actually looked at arrow syntax for function signatures. I don't remember the details but I'm pretty sure notation such as

func f (x int) -> float32

was on the white board. Eventually we dropped the arrow because it didn't work that well with multiple (non-tuple) return values; and once the func and the parameters where present, the arrow was superfluous; perhaps "pretty" (as in mathematically looking), but still superfluous. It also seemed like syntax that belonged to a "different" kind of language.

But having closures in a performant, general purpose language opened the doors to new, more functional programming styles. Now, 10 years down the road, one might look at it from a different angle.

Still, I think we have to be very careful here to not create special syntax for closures. What we have now is simple and regular and has worked well so far. Whatever the approach, if there's any change, I believe it will need to be regular and apply to any function.

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commented Jan 3, 2018

In Go, the syntax for function declarations deviates a bit from the regular pattern that we have for other declarations. For constants, types, variables we always have:
keyword name type value
[…]
For functions this breaks down, the type always must be a literal signature.

Note that for parameter lists and const and var declarations we have a similar pattern, IdentifierList Type, which we should probably also preserve. That seems like it would rule out the lambda-calculus-style : token to separate variable names from types.

Whatever the approach, if there's any change, I believe it will need to be regular and apply to any function.

The keyword name type value pattern is for declarations, but the use-cases that @neild mentions are all for literals.

If we address the problem of literals, then I believe the problem of declarations becomes trivial. For declarations of constants, variables, and now types, we allow (or require) an = token before the value. It seems like it would be easy enough to extend that to functions:

FunctionDecl = "func" ( FunctionSpec | "(" { FunctionSpec ";" } ")" ).
FunctionSpec = FunctionName Function |
               IdentifierList (Signature | [ Signature ] "=" Expression) .

FunctionLit = "func" Function | ShortFunctionLit .
ShortParameterList = ShortParameterDecl { "," ShortParameterDecl } .
ShortParameterDecl = IdentifierList [ "..." ] [ Type ] .

The expression after the = token must be a function literal, or perhaps a function returned by a call whose arguments are all available at compile time. In the = form, a Signature could still be supplied to move the argument type declarations from the literal to the FunctionSpec.

Note that the difference between a ShortParameterDecl and the existing ParameterDecl is that singleton IdentifierLists are interpreted as parameter names instead of types.


Examples

Consider this function declaration accepted today:

func compute(f func(x, y float64) float64) float64 { return f(3, 4) }

We could either retain that (e.g. for Go 1 compatibility) in addition to the examples below, or eliminate the Function production and use only the ShortFunctionLit version.

For various ShortFunctionLit options, the grammar I propose above gives:

Rust-like:

ShortFunctionLit = "|" ShortParameterList "|" Block .

Admits any of:

func compute = |f func(x, y float64) float64| { f(3, 4) }
func compute(func (x, y float64) float64) float64 = |f| { f(3, 4) }
func (
	compute = |f func(x, y float64) float64| { f(3, 4) }
)
func (
	compute(func (x, y float64) float64) float64 = |f| { f(3, 4) }
)

Scala-like:

ShortFunctionLit = "(" ShortParameterList ")" "=>" Expression .

Admits any of:

func compute = (f func(x, y float64) float64) => f(3, 4)
func compute(func (x, y float64) float64) float64 = (f) => f(3, 4)
func (
	compute = (f func(x, y float64) float64) => f(3, 4)
)
func (
	compute(func (x, y float64) float64) float64 = (f) => f(3, 4)
)

Lambda-calculus-like:

ShortFunctionLit = "λ" ShortParameterList "." Expression .

Admits any of:

func compute = λf func(x, y float64) float64.f(3, 4)
func compute(func (x, y float64) float64) float64) = λf.f(3, 4)
func (
	compute = λf func(x, y float64) float64.f(3, 4)
)
func (
	compute(func (x, y float64) float64) float64) = λf.f(3, 4)
)

Haskell-like:

ShortFunctionLit = "\" ShortParameterList "->" Expression .
func compute = \f func(x, y float64) float64 -> f(3, 4)
func compute(func (x, y float64) float64) float64) = \f -> f(3, 4)
func (
	compute = \f func(x, y float64) float64 -> f(3, 4)
)
func (
	compute(func (x, y float64) float64) float64) = \f -> f(3, 4)
)

C++-like:
(Probably not feasible due to ambiguity with array literals, but maybe worth considering.)

ShortFunctionLit = "[" ShortParameterList "]" Block .

Admits any of:

func compute = [f func(x, y float64) float64] { f(3, 4) }
func compute(func (x, y float64) float64) float64) = [f] { f(3, 4) }
func (
	compute = [f func(x, y float64) float64] { f(3, 4) }
)
func (
	compute(func (x, y float64) float64) float64) = [f] { f(3, 4) }
)

Personally, I find all but the Scala-like variants to be fairly legible. (To my eye, the Scala-like variant is too heavy on parentheses: it makes the lines much more difficult to scan.)

@ianlancetaylor

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commented Jan 3, 2018

Personally I'm mainly interested in this if it lets me omit the parameter and result types when they can be inferred. I'm even fine with the current function literal syntax if I can do that. (This was discussed above.)

Admittedly this goes against @griesemer 's comment.

@neild

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commented Jan 3, 2018

Whatever the approach, if there's any change, I believe it will need to be regular and apply to any function.

I don't quite follow this. Function declarations necessarily must include the full type information for the function, since there's no way to derive it with sufficient precision from the function body. (This isn't the case for all languages, of course, but it is for Go.)

Function literals, in contrast, could infer type information from context.

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commented Jan 3, 2018

@neild Apologies for being imprecise: What I meant this sentence is that if there were new different syntax (arrows or what have you), it should be somewhat regular and apply everywhere. If it's possible that types can be omitted, that would be again orthogonal.

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commented Jan 3, 2018

@griesemer Thanks; I (mostly) agree with that point.

I think the interesting question for this proposal is whether having some syntax is a good idea or not; what that syntax would be is important but relatively trivial.

However, I can't resist the temptation to bikeshed my own proposal a bit.

var sum func(int, int) int = func a, b { return a + b }
@firelizzard18

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commented Jan 4, 2018

@neild's proposal feels right to me. It's pretty close to the existing syntax, but works for functional programming as it eliminates the repetition of the type specifications. It's not that much less compact than (a, b) => a + b, and it fits well into the existing syntax.

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commented Jan 4, 2018

@neild

var sum func(int, int) int = func a, b { return a + b }

Would that declare a variable, or a function? If a variable, what would the equivalent function declaration look like?

Under my declaration schema above, if I'm understanding correctly it would be:

ShortFunctionLit = "func" ShortParameterList Block .
func compute = func f func(x, y float64) float64 { return f(3, 4) }
func compute(func (x, y float64) float64) float64 = func f { return f(3, 4) }
func (
	compute = func f func(x, y float64) float64 { return f(3, 4) }
)
func (
	compute(func (x, y float64) float64) float64 = func f { return f(3, 4) }
)

I don't think I'm a fan: it stutters a bit on func, and doesn't seem to provide enough of a visual break between the func token and the parameters that follow.

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commented Jan 4, 2018

Or would you leave out parens from the declaration, rather than assigning to literals?

func compute f func(x, y float64) float64 { return f(3, 4) }

I still don't like the lack of visual break, though...

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commented Jan 4, 2018

Would that declare a variable, or a function? If a variable, what would the equivalent function declaration look like?

A variable. The equivalent function declaration would presumably be func sum a, b { return a+b }, but that would be invalid for obvious reasons--you can't elide parameter types from function declarations.

The grammar change I'm thinking of would be something like:

ShortFunctionLit = "func" [ IdentifierList ] [ "..." ] FunctionBody .

A short function literal is distinguished from a regular function literal by omitting the parentheses on the parameter list, defines only the names of the incoming parameters, and does not define the outgoing parameters. The types of the incoming parameters and the types and number of outgoing parameters are derived from the surrounding context.

I don't think there's any need to allow specifying optional parameter types in a short function literal; you just use a regular function literal in that case.

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commented Jan 4, 2018

As @ianlancetaylor pointed out, the light-weight notation really only makes sense when it permits the omission of parameter types because they can be inferred easily. As such, @neild 's suggestion is the best and simplest I've seen so far. The one thing it doesn't permit easily though is a light-weight notation for function literals that want to refer to named result parameters. But perhaps in that case they should use the full notation. (It's just a bit irregular).

We might even be able to parse (x, y) { ... } as short form for func (x, y T) T { ... }; though it would require a bit of parser look-ahead, but perhaps not too bad.

@neild

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commented Jan 4, 2018

As an experiment, I modified gofmt to rewrite function literals into the compact syntax and ran it against src/. You can see the results here:

neild@2ff18c6

I didn't make any attempt to limit this to cases where it makes sense; I just wanted to get a sense for how the compact syntax might play out in practice. I haven't dug through it enough yet to develop any opinions on the results.

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commented Jan 5, 2018

@neild Nice analysis. Some observations:

  1. The fraction of cases in which the function literal is bound using := is disappointing, since handling those cases without explicit type annotations would require a more complicated inference algorithm.

  2. The literals passed to callbacks are easier to read in some cases, but more difficult in others.
    For example, losing the return-type information for function literals that span many lines is a bit unfortunate, since that also tells the reader whether they're looking at a functional API or an imperative one.

  3. The reduction in boilerplate for function literals within slices is substantial.

  4. defer and go statements are an interesting case: would we infer the argument types from the arguments actually passed to the function?

  5. A couple of trailing ... tokens are missing from the examples.

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commented Jan 5, 2018

defer and go are indeed a quite interesting case.

go func p {
  // do something with p
}("parameter")

Would we derive the type of p from the actual function parameter? This would be quite nice for go statements, although you can of course achieve much the same effect by just using a closure:

p := "parameter"
go func() {
  // do something with p
}()
@fbnz156

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commented May 10, 2018

I would totally support this. Frankly I don't care how much it "looks like other languages", I just want a less verbose way to use anonymous functions.

@networkimprov

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commented Sep 20, 2018

EDIT: Borrowing the composite literal syntax...

type F func(int) float64
var f F
f = F {      (i) (o) { o = float64(i); return } }
f = F {      (i) o   { o = float64(i); return } } // single return value
f = F { func (i) o   { o = float64(i); return } } // +func for good measure?
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commented Nov 11, 2018

I can see writing a large amount of anonymous functions with current syntax would get annoying.

I like Go because most code flows and looks the same, and there is not many decisions to make, ie formatting, space vs tab, etc..

I don't like "adding" another way to write anonymous function. If new this way is added, please remove the old way so all code will still look the same. If removing the old way is not possible, please do not add a new way.

@vp2177

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commented Jan 31, 2019

Just an idea:
Here is what OP's example would look like with an untyped function literal with Swift's syntax:

compute({ $0 + $1 })

I believe this would have the advantage of being fully backwards compatible with Go 1.

@UlisseMini

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commented May 14, 2019

I just found this because i was writing a simple tcp chat app,
basically i have a structure with a slice inside it

type connIndex struct {
	conns []net.Conn
	mu    sync.Mutex
}

and i'd like to apply some operations to it concurrently (adding connections, sending messages to all etc)

and instead of following the normal path of copy-pasting the mutex locking code, or using a daemon goroutine to manage access i thought i'd just pass a closure

func (c *connIndex) run(f func([]net.Conn)) {
	c.mu.Lock()
	defer c.mu.Unlock()
	f(c.conns)
}

for short operations its overly verbose (still better then lock and defer unlock())

conns.run(func(conns []net.Conn) { conns = append(conns, conn) })

This violates the DRY principle as i've typed out that exact function signature in the run method.

If go supported infering the function signature i could write it like this

conns.run(func(conns) { conns = append(conns, conn) })

I don't think this makes the code less readable, you can tell it is a slice because of append, and because i've named my variables well you can guess it is a []net.Conn without looking at the run method signature.

I'd avoid trying to infer the types of paramaters based on the function body, instead add inference only for cases where it is obvious (like passing closures to functions).

i'd say this does not harm readibility as it gives the reader an option, if they don't know the type of the paramater they can godef it or hover over it and get the editor to show it to them.

Sorta like how in a book they don't repeat the characters introduction, except we would have a button to show it / jump to it.

I'm bad at writing so hopefully you survived reading this :)

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