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cmd/compile: should auto dereference <pointer-to-map>[index] #44211

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docwhat opened this issue Feb 10, 2021 · 4 comments
Closed

cmd/compile: should auto dereference <pointer-to-map>[index] #44211

docwhat opened this issue Feb 10, 2021 · 4 comments

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@docwhat
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@docwhat docwhat commented Feb 10, 2021

What version of Go are you using (go version)?

$ go version
go version go1.15.7 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

go env Output
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/docwhat/Library/Caches/go-build"
GOENV="/Users/docwhat/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/docwhat/go/pkg/mod"
GOOS="darwin"
GOPATH="/Users/docwhat/go"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.15.7_1/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.15.7_1/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/wq/4zytjdrx2l7f5xf6vl7k56340000gn/T/go-build573187384=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I wrote code that looked like this:

type Currency string

type Balance map[Currency]float32

func (b *Balance) Add(amount Amount) *Balance {
    current, ok := b[amount.Currency]
    if ok {
        b[amount.Currency] = current + amount.Value
    } else {
        b[amount.Currency] = amount.Value
    }
    return b
}

What did you expect to see?

I expected it to work.

What did you see instead?

prog.go:15: invalid operation: b[amount.Currency] (type *Balance does not support indexing)
prog.go:17: invalid operation: b[amount.Currency] (type *Balance does not support indexing)
prog.go:19: invalid operation: b[amount.Currency] (type *Balance does not support indexing)

Additional info

Changing the code to:

func (b *Balance) Add(amount Amount) *Balance {
    current, ok := (*b)[amount.Currency]
    if ok {
        (*b)[amount.Currency] = current + amount.Value
    } else {
        (*b)[amount.Currency] = amount.Value
    }
    return b
}

Since the compiler automatically dereferences *structs in other places, why doesn't it dereference b[key] as well? Does it not have enough information?

The source code was gotten from StackOverflow: Go: invalid operation - type *map[key]value does not support indexing. I have some similar though more complicated code.

@cherrymui
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@cherrymui cherrymui commented Feb 10, 2021

What is the type of Balance? Index operation never works on struct, or *struct.

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@dmitshur dmitshur changed the title compiler should auto dereference <pointer-to-struct>[index] cmd/compile: should auto dereference <pointer-to-struct>[index] Feb 10, 2021
@docwhat
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@docwhat docwhat commented Feb 10, 2021

What is the type of Balance?

type Balance map[Currency]float32

Index operation never works on struct, or *struct.

It doesn't work, true. But why not? Doesn't the compiler have enough information?

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@randall77
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@randall77 randall77 commented Feb 10, 2021

The compiler only implicitly dereferences on pointers to arrays, not pointers to maps (or other types, like slices). So if you had

type Balance [4]float32

it would work.

I don't think it makes sense to autodereference in this case. It is seldom the right answer to have a pointer to a map, so there's no point in providing syntactic sugar for it.
(There's no point in your Add method to take a *Balance receiver. Just take a Balance.)

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@randall77 randall77 changed the title cmd/compile: should auto dereference <pointer-to-struct>[index] cmd/compile: should auto dereference <pointer-to-map>[index] Feb 10, 2021
@ianlancetaylor
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@ianlancetaylor ianlancetaylor commented Feb 10, 2021

This is not a compiler issue. This is a language issue. The language does not permit the compiler to make this automatic dereference. Automatic dereference is only defined for selectors (https://golang.org/ref/spec#Selectors) and for array (not slice) indexes (https://golang.org/ref/spec#Index_expressions). Closing this issue.

(You could file a language change proposal for this -- see https://golang.org/s/proposal -- but as pointer-to-map types are rarely used I don't think there would be much support for such a change.)

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5 participants