Assigning slices of equivalent types doesn't work

-3

Why doesn't this work?

package main



type Word uint8

type Memory Word



func main() {

    bytes := uint8{}

    memory := Memory{}

    bytes = memory

}

The compiler generates this error:

9:9: cannot use memory (type Memory) as type byte in assignment

As I understand it, uint8 and Memory should be mutually assignable.

edited Nov 20 '18 at 16:20

RayfenWindspear

3,6431229

asked Nov 19 '18 at 23:33

Martín Straus

41436

Took a second look after your comment to zerkms a few minutes ago. There is more going on here than what it seems. For some reason, even a uint8 is not assignable to a Word (and vice versa). play.golang.org/p/BrU8mz4ZdUh

– RayfenWindspear
Nov 20 '18 at 16:42

Possible duplicate of Assignability of function parameters in golang

– RayfenWindspear
Nov 20 '18 at 17:08

Discovered the subtlety :)

– RayfenWindspear
Nov 20 '18 at 17:09

This seems to go into quite a bit of detail. go101.org/article/…

– RayfenWindspear
Nov 20 '18 at 17:17

add a comment |

-3

Why doesn't this work?

package main



type Word uint8

type Memory Word



func main() {

    bytes := uint8{}

    memory := Memory{}

    bytes = memory

}

The compiler generates this error:

9:9: cannot use memory (type Memory) as type byte in assignment

As I understand it, uint8 and Memory should be mutually assignable.

edited Nov 20 '18 at 16:20

RayfenWindspear

3,6431229

asked Nov 19 '18 at 23:33

Martín Straus

41436

Took a second look after your comment to zerkms a few minutes ago. There is more going on here than what it seems. For some reason, even a uint8 is not assignable to a Word (and vice versa). play.golang.org/p/BrU8mz4ZdUh

– RayfenWindspear
Nov 20 '18 at 16:42

Possible duplicate of Assignability of function parameters in golang

– RayfenWindspear
Nov 20 '18 at 17:08

Discovered the subtlety :)

– RayfenWindspear
Nov 20 '18 at 17:09

This seems to go into quite a bit of detail. go101.org/article/…

– RayfenWindspear
Nov 20 '18 at 17:17

add a comment |

-3

Why doesn't this work?

package main



type Word uint8

type Memory Word



func main() {

    bytes := uint8{}

    memory := Memory{}

    bytes = memory

}

The compiler generates this error:

9:9: cannot use memory (type Memory) as type byte in assignment

As I understand it, uint8 and Memory should be mutually assignable.

edited Nov 20 '18 at 16:20

RayfenWindspear

3,6431229

asked Nov 19 '18 at 23:33

Martín Straus

41436

Why doesn't this work?

package main



type Word uint8

type Memory Word



func main() {

    bytes := uint8{}

    memory := Memory{}

    bytes = memory

}

The compiler generates this error:

9:9: cannot use memory (type Memory) as type byte in assignment

As I understand it, uint8 and Memory should be mutually assignable.

edited Nov 20 '18 at 16:20

RayfenWindspear

3,6431229

asked Nov 19 '18 at 23:33

Martín Straus

41436

edited Nov 20 '18 at 16:20

RayfenWindspear

3,6431229

asked Nov 19 '18 at 23:33

Martín Straus

41436

edited Nov 20 '18 at 16:20

RayfenWindspear

3,6431229

edited Nov 20 '18 at 16:20

RayfenWindspear

3,6431229

edited Nov 20 '18 at 16:20

RayfenWindspear

3,6431229

asked Nov 19 '18 at 23:33

Martín Straus

41436

asked Nov 19 '18 at 23:33

Martín Straus

41436

asked Nov 19 '18 at 23:33

Martín Straus

41436

Took a second look after your comment to zerkms a few minutes ago. There is more going on here than what it seems. For some reason, even a uint8 is not assignable to a Word (and vice versa). play.golang.org/p/BrU8mz4ZdUh

– RayfenWindspear
Nov 20 '18 at 16:42

Possible duplicate of Assignability of function parameters in golang

– RayfenWindspear
Nov 20 '18 at 17:08

Discovered the subtlety :)

– RayfenWindspear
Nov 20 '18 at 17:09

This seems to go into quite a bit of detail. go101.org/article/…

– RayfenWindspear
Nov 20 '18 at 17:17

add a comment |

Took a second look after your comment to zerkms a few minutes ago. There is more going on here than what it seems. For some reason, even a uint8 is not assignable to a Word (and vice versa). play.golang.org/p/BrU8mz4ZdUh

– RayfenWindspear
Nov 20 '18 at 16:42

Possible duplicate of Assignability of function parameters in golang

– RayfenWindspear
Nov 20 '18 at 17:08

Discovered the subtlety :)

– RayfenWindspear
Nov 20 '18 at 17:09

This seems to go into quite a bit of detail. go101.org/article/…

– RayfenWindspear
Nov 20 '18 at 17:17

Took a second look after your comment to zerkms a few minutes ago. There is more going on here than what it seems. For some reason, even a uint8 is not assignable to a Word (and vice versa). play.golang.org/p/BrU8mz4ZdUh

– RayfenWindspear
Nov 20 '18 at 16:42

Possible duplicate of Assignability of function parameters in golang

– RayfenWindspear
Nov 20 '18 at 17:08

Discovered the subtlety :)

– RayfenWindspear
Nov 20 '18 at 17:09

This seems to go into quite a bit of detail. go101.org/article/…

– RayfenWindspear
Nov 20 '18 at 17:17

add a comment |

1 Answer
1

active

oldest

votes

Here are the assignability rules

In this particular case none of those is held, so the types are not assignable.

Given you mentioned this answer is not detailed enough - let's run through every assignability rule: (for ease let's use A as a substitute for uint8 and B as a substitute for Word)

x's type is identical to T. --- Nope, A is not identical to B

x's type V and T have identical underlying types and at least one of V or T is not a defined type. --- Nope, both are defined types

T is an interface type and x implements T. --- None of them is an interface

x is a bidirectional channel value, T is a channel type, x's type V and T have identical element types, and at least one of V or T is not a defined type. --- None of them is a channel

x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type. --- None of them holds nil value

x is an untyped constant representable by a value of type T. --- Nope, no constants involved

So, as you can see - there is no way A is assignable to B.

If you declare Memory as type Memory uint8 it would work (as @RayfenWindspear mentioned in the comments), I'm sure though the question roots is more "why" not "how to fix".

edited Nov 20 '18 at 19:48

answered Nov 19 '18 at 23:40

zerkms

187k48339430

1

Explained in the spec, but it's useful to note for this specific instance, cutting out Word and just using uint8 makes them assignable.

– RayfenWindspear
Nov 19 '18 at 23:42

@RayfenWindspear indeed, added it, thanks

– zerkms
Nov 19 '18 at 23:46

My comment (which could have been better) was to illustrate that this is a case of a = b and b = c but with Go assignability of types, that doesn't mean a = c.

– RayfenWindspear
Nov 19 '18 at 23:51

@RayfenWindspear it's probably even a bit more complicated because of covariance (lack of it)

– zerkms
Nov 20 '18 at 0:05

I read the assignability rules before posting, but I can't figure out why in that specific case uint8 is not equal to Word. Plus, I specifically asked "Why", not "how to fix". Thus, your answer isn't enought.

– Martín Straus
Nov 20 '18 at 16:14

|
show 1 more comment

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1 Answer
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Here are the assignability rules

In this particular case none of those is held, so the types are not assignable.

Given you mentioned this answer is not detailed enough - let's run through every assignability rule: (for ease let's use A as a substitute for uint8 and B as a substitute for Word)

x's type is identical to T. --- Nope, A is not identical to B

x's type V and T have identical underlying types and at least one of V or T is not a defined type. --- Nope, both are defined types

T is an interface type and x implements T. --- None of them is an interface

x is a bidirectional channel value, T is a channel type, x's type V and T have identical element types, and at least one of V or T is not a defined type. --- None of them is a channel

x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type. --- None of them holds nil value

x is an untyped constant representable by a value of type T. --- Nope, no constants involved

So, as you can see - there is no way A is assignable to B.

If you declare Memory as type Memory uint8 it would work (as @RayfenWindspear mentioned in the comments), I'm sure though the question roots is more "why" not "how to fix".

edited Nov 20 '18 at 19:48

answered Nov 19 '18 at 23:40

zerkms

187k48339430

1

Explained in the spec, but it's useful to note for this specific instance, cutting out Word and just using uint8 makes them assignable.

– RayfenWindspear
Nov 19 '18 at 23:42

@RayfenWindspear indeed, added it, thanks

– zerkms
Nov 19 '18 at 23:46

My comment (which could have been better) was to illustrate that this is a case of a = b and b = c but with Go assignability of types, that doesn't mean a = c.

– RayfenWindspear
Nov 19 '18 at 23:51

@RayfenWindspear it's probably even a bit more complicated because of covariance (lack of it)

– zerkms
Nov 20 '18 at 0:05

I read the assignability rules before posting, but I can't figure out why in that specific case uint8 is not equal to Word. Plus, I specifically asked "Why", not "how to fix". Thus, your answer isn't enought.

– Martín Straus
Nov 20 '18 at 16:14

|
show 1 more comment

Here are the assignability rules

In this particular case none of those is held, so the types are not assignable.

Given you mentioned this answer is not detailed enough - let's run through every assignability rule: (for ease let's use A as a substitute for uint8 and B as a substitute for Word)

x's type is identical to T. --- Nope, A is not identical to B

x's type V and T have identical underlying types and at least one of V or T is not a defined type. --- Nope, both are defined types

T is an interface type and x implements T. --- None of them is an interface

x is a bidirectional channel value, T is a channel type, x's type V and T have identical element types, and at least one of V or T is not a defined type. --- None of them is a channel

x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type. --- None of them holds nil value

x is an untyped constant representable by a value of type T. --- Nope, no constants involved

So, as you can see - there is no way A is assignable to B.

If you declare Memory as type Memory uint8 it would work (as @RayfenWindspear mentioned in the comments), I'm sure though the question roots is more "why" not "how to fix".

edited Nov 20 '18 at 19:48

answered Nov 19 '18 at 23:40

zerkms

187k48339430

1

Explained in the spec, but it's useful to note for this specific instance, cutting out Word and just using uint8 makes them assignable.

– RayfenWindspear
Nov 19 '18 at 23:42

@RayfenWindspear indeed, added it, thanks

– zerkms
Nov 19 '18 at 23:46

My comment (which could have been better) was to illustrate that this is a case of a = b and b = c but with Go assignability of types, that doesn't mean a = c.

– RayfenWindspear
Nov 19 '18 at 23:51

@RayfenWindspear it's probably even a bit more complicated because of covariance (lack of it)

– zerkms
Nov 20 '18 at 0:05

I read the assignability rules before posting, but I can't figure out why in that specific case uint8 is not equal to Word. Plus, I specifically asked "Why", not "how to fix". Thus, your answer isn't enought.

– Martín Straus
Nov 20 '18 at 16:14

|
show 1 more comment

Here are the assignability rules

In this particular case none of those is held, so the types are not assignable.

Given you mentioned this answer is not detailed enough - let's run through every assignability rule: (for ease let's use A as a substitute for uint8 and B as a substitute for Word)

x's type is identical to T. --- Nope, A is not identical to B

x's type V and T have identical underlying types and at least one of V or T is not a defined type. --- Nope, both are defined types

T is an interface type and x implements T. --- None of them is an interface

x is a bidirectional channel value, T is a channel type, x's type V and T have identical element types, and at least one of V or T is not a defined type. --- None of them is a channel

x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type. --- None of them holds nil value

x is an untyped constant representable by a value of type T. --- Nope, no constants involved

So, as you can see - there is no way A is assignable to B.

If you declare Memory as type Memory uint8 it would work (as @RayfenWindspear mentioned in the comments), I'm sure though the question roots is more "why" not "how to fix".

edited Nov 20 '18 at 19:48

answered Nov 19 '18 at 23:40

zerkms

187k48339430

Here are the assignability rules

In this particular case none of those is held, so the types are not assignable.

Given you mentioned this answer is not detailed enough - let's run through every assignability rule: (for ease let's use A as a substitute for uint8 and B as a substitute for Word)

x's type is identical to T. --- Nope, A is not identical to B

x's type V and T have identical underlying types and at least one of V or T is not a defined type. --- Nope, both are defined types

T is an interface type and x implements T. --- None of them is an interface

x is a bidirectional channel value, T is a channel type, x's type V and T have identical element types, and at least one of V or T is not a defined type. --- None of them is a channel

x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type. --- None of them holds nil value

x is an untyped constant representable by a value of type T. --- Nope, no constants involved

So, as you can see - there is no way A is assignable to B.

If you declare Memory as type Memory uint8 it would work (as @RayfenWindspear mentioned in the comments), I'm sure though the question roots is more "why" not "how to fix".

edited Nov 20 '18 at 19:48

answered Nov 19 '18 at 23:40

zerkms

187k48339430

edited Nov 20 '18 at 19:48

answered Nov 19 '18 at 23:40

zerkms

187k48339430

answered Nov 19 '18 at 23:40

zerkms

187k48339430

answered Nov 19 '18 at 23:40

zerkms

187k48339430

1

Explained in the spec, but it's useful to note for this specific instance, cutting out Word and just using uint8 makes them assignable.

– RayfenWindspear
Nov 19 '18 at 23:42

@RayfenWindspear indeed, added it, thanks

– zerkms
Nov 19 '18 at 23:46

My comment (which could have been better) was to illustrate that this is a case of a = b and b = c but with Go assignability of types, that doesn't mean a = c.

– RayfenWindspear
Nov 19 '18 at 23:51

@RayfenWindspear it's probably even a bit more complicated because of covariance (lack of it)

– zerkms
Nov 20 '18 at 0:05

I read the assignability rules before posting, but I can't figure out why in that specific case uint8 is not equal to Word. Plus, I specifically asked "Why", not "how to fix". Thus, your answer isn't enought.

– Martín Straus
Nov 20 '18 at 16:14

|
show 1 more comment

1

Explained in the spec, but it's useful to note for this specific instance, cutting out Word and just using uint8 makes them assignable.

– RayfenWindspear
Nov 19 '18 at 23:42

@RayfenWindspear indeed, added it, thanks

– zerkms
Nov 19 '18 at 23:46

My comment (which could have been better) was to illustrate that this is a case of a = b and b = c but with Go assignability of types, that doesn't mean a = c.

– RayfenWindspear
Nov 19 '18 at 23:51

@RayfenWindspear it's probably even a bit more complicated because of covariance (lack of it)

– zerkms
Nov 20 '18 at 0:05

I read the assignability rules before posting, but I can't figure out why in that specific case uint8 is not equal to Word. Plus, I specifically asked "Why", not "how to fix". Thus, your answer isn't enought.

– Martín Straus
Nov 20 '18 at 16:14

Explained in the spec, but it's useful to note for this specific instance, cutting out Word and just using uint8 makes them assignable.

– RayfenWindspear
Nov 19 '18 at 23:42

@RayfenWindspear indeed, added it, thanks

– zerkms
Nov 19 '18 at 23:46

My comment (which could have been better) was to illustrate that this is a case of a = b and b = c but with Go assignability of types, that doesn't mean a = c.

– RayfenWindspear
Nov 19 '18 at 23:51

@RayfenWindspear it's probably even a bit more complicated because of covariance (lack of it)

– zerkms
Nov 20 '18 at 0:05

I read the assignability rules before posting, but I can't figure out why in that specific case uint8 is not equal to Word. Plus, I specifically asked "Why", not "how to fix". Thus, your answer isn't enought.

– Martín Straus
Nov 20 '18 at 16:14

|
show 1 more comment

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