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Using Promise as return type in Flow

Flow Types with Promises (Fetch's)Get the name of an object's typeevent.preventDefault() vs. return falseWhy does ++[[]][+[]]+[+[]] return the string “10”?How do I return the response from an asynchronous call?declaration of flow (flowtype) generic typeError: Flow: WeakMap. Application of polymorphic type needs <list of 2 arguments>. (Can use `*` for inferrable ones)Flow errors on PromisesFlow type return type for async stringFlow does keeps interpreting variables as Promises after async/awaitMomentjs and Flow types

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24

I'm using Flow 0.30.0. I have a function that returns a promise:

function process(callback:Function):Promise 
 return new Promise((r,re) => callback)

Flow raises an error, complaining:

function process(callback:Function):Promise {
 ^^^^^^^ Promise. Application of polymorphic type needs <list of 1 argument>. (Can use `*` for inferrable ones)

This works in Try Flow, but not locally. Any ideas of the issue?

javascript flowtype

share|improve this question

edited Aug 20 '18 at 19:10

Rory O'Kane

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asked Aug 2 '16 at 21:20

Allyl IsocyanateAllyl Isocyanate

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add a comment
 | 

24

I'm using Flow 0.30.0. I have a function that returns a promise:

function process(callback:Function):Promise 
 return new Promise((r,re) => callback)

Flow raises an error, complaining:

function process(callback:Function):Promise {
 ^^^^^^^ Promise. Application of polymorphic type needs <list of 1 argument>. (Can use `*` for inferrable ones)

This works in Try Flow, but not locally. Any ideas of the issue?

javascript flowtype

share|improve this question

edited Aug 20 '18 at 19:10

Rory O'Kane

19.9k88 gold badges6868 silver badges110110 bronze badges

asked Aug 2 '16 at 21:20

Allyl IsocyanateAllyl Isocyanate

6,7641313 gold badges5656 silver badges108108 bronze badges

add a comment
 | 

I'm using Flow 0.30.0. I have a function that returns a promise:

function process(callback:Function):Promise 
 return new Promise((r,re) => callback)

Flow raises an error, complaining:

function process(callback:Function):Promise {
 ^^^^^^^ Promise. Application of polymorphic type needs <list of 1 argument>. (Can use `*` for inferrable ones)

This works in Try Flow, but not locally. Any ideas of the issue?

javascript flowtype

share|improve this question

edited Aug 20 '18 at 19:10

Rory O'Kane

19.9k88 gold badges6868 silver badges110110 bronze badges

asked Aug 2 '16 at 21:20

Allyl IsocyanateAllyl Isocyanate

6,7641313 gold badges5656 silver badges108108 bronze badges

function process(callback:Function):Promise 
 return new Promise((r,re) => callback)

function process(callback:Function):Promise {
 ^^^^^^^ Promise. Application of polymorphic type needs <list of 1 argument>. (Can use `*` for inferrable ones)

share|improve this question

edited Aug 20 '18 at 19:10

Rory O'Kane

19.9k88 gold badges6868 silver badges110110 bronze badges

asked Aug 2 '16 at 21:20

Allyl IsocyanateAllyl Isocyanate

6,7641313 gold badges5656 silver badges108108 bronze badges

share|improve this question

edited Aug 20 '18 at 19:10

Rory O'Kane

19.9k88 gold badges6868 silver badges110110 bronze badges

asked Aug 2 '16 at 21:20

Allyl IsocyanateAllyl Isocyanate

6,7641313 gold badges5656 silver badges108108 bronze badges

edited Aug 20 '18 at 19:10

Rory O'Kane

19.9k88 gold badges6868 silver badges110110 bronze badges

edited Aug 20 '18 at 19:10

Rory O'Kane

19.9k88 gold badges6868 silver badges110110 bronze badges

asked Aug 2 '16 at 21:20

Allyl IsocyanateAllyl Isocyanate

6,7641313 gold badges5656 silver badges108108 bronze badges

asked Aug 2 '16 at 21:20

Allyl IsocyanateAllyl Isocyanate

6,7641313 gold badges5656 silver badges108108 bronze badges

2 Answers
2

active

oldest

votes

37

According to the flow changelog there was a change introduced in v0.28.0 which made type arguments explicit. Explicitly setting the return type to Promise<any> should fix your issue.

Alternatively you can set the following flag to false, although this is a temporary flag which will be removed in the future

experimental.strict_type_args=false

https://github.com/facebook/flow/blob/master/Changelog.md#v0280

share|improve this answer

answered Aug 2 '16 at 21:34

mike dmike d

77911 gold badge66 silver badges1515 bronze badges

• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  Excuse my ignorance but I've been trying to find the Promise type documentation in Flow Docs (flow.org/en/docs/types) but I couldn't find anything. Where is it documented?
  
  – Leonardo
  Sep 13 '17 at 23:23
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Leonardo I am in the same situation, have you found the docs on it?
  
  – Noitidart
  Sep 18 '17 at 5:52
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Noitidart kind of, I had to open an issue on github where they pointed me to the code implementing the Promise Type. Apparently the code exists but it's not documented :
  
  – Leonardo
  Sep 20 '17 at 9:27
  
  

  
  
  
  

add a comment
 | 

0

I'm guessing this was figured some time ago and the correct answer to what was going on was that it was a versions mismatch between local and the on-line flow interpreter.

However, as this question turns up near the top of search results when looking for how to Flow annotate Promises, it's probably worth mentioning that the fix for these types of situations is to specify the Flow type that the Promise resolves to as a Flow angle bracket parameter.

In this case adding a "any" to the Promise annotation silences the error e.g. ...

function process(callback:Function):Promise<any> 
 return new Promise((r,re) => callback)

In the docs over here as well for another example

share|improve this answer

edited Apr 3 at 10:38

answered Mar 28 at 10:25

shufflingbshufflingb

5166 bronze badges

add a comment
 | 

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37

According to the flow changelog there was a change introduced in v0.28.0 which made type arguments explicit. Explicitly setting the return type to Promise<any> should fix your issue.

Alternatively you can set the following flag to false, although this is a temporary flag which will be removed in the future

experimental.strict_type_args=false

https://github.com/facebook/flow/blob/master/Changelog.md#v0280

share|improve this answer

answered Aug 2 '16 at 21:34

mike dmike d

77911 gold badge66 silver badges1515 bronze badges

• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  Excuse my ignorance but I've been trying to find the Promise type documentation in Flow Docs (flow.org/en/docs/types) but I couldn't find anything. Where is it documented?
  
  – Leonardo
  Sep 13 '17 at 23:23
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Leonardo I am in the same situation, have you found the docs on it?
  
  – Noitidart
  Sep 18 '17 at 5:52
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Noitidart kind of, I had to open an issue on github where they pointed me to the code implementing the Promise Type. Apparently the code exists but it's not documented :
  
  – Leonardo
  Sep 20 '17 at 9:27
  
  

  
  
  
  

add a comment
 | 

37

According to the flow changelog there was a change introduced in v0.28.0 which made type arguments explicit. Explicitly setting the return type to Promise<any> should fix your issue.

Alternatively you can set the following flag to false, although this is a temporary flag which will be removed in the future

experimental.strict_type_args=false

https://github.com/facebook/flow/blob/master/Changelog.md#v0280

share|improve this answer

answered Aug 2 '16 at 21:34

mike dmike d

77911 gold badge66 silver badges1515 bronze badges

• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  Excuse my ignorance but I've been trying to find the Promise type documentation in Flow Docs (flow.org/en/docs/types) but I couldn't find anything. Where is it documented?
  
  – Leonardo
  Sep 13 '17 at 23:23
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Leonardo I am in the same situation, have you found the docs on it?
  
  – Noitidart
  Sep 18 '17 at 5:52
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Noitidart kind of, I had to open an issue on github where they pointed me to the code implementing the Promise Type. Apparently the code exists but it's not documented :
  
  – Leonardo
  Sep 20 '17 at 9:27
  
  

  
  
  
  

add a comment
 | 

According to the flow changelog there was a change introduced in v0.28.0 which made type arguments explicit. Explicitly setting the return type to Promise<any> should fix your issue.

Alternatively you can set the following flag to false, although this is a temporary flag which will be removed in the future

experimental.strict_type_args=false

https://github.com/facebook/flow/blob/master/Changelog.md#v0280

share|improve this answer

answered Aug 2 '16 at 21:34

mike dmike d

77911 gold badge66 silver badges1515 bronze badges

share|improve this answer

answered Aug 2 '16 at 21:34

mike dmike d

77911 gold badge66 silver badges1515 bronze badges

answered Aug 2 '16 at 21:34

mike dmike d

77911 gold badge66 silver badges1515 bronze badges

answered Aug 2 '16 at 21:34

mike dmike d

77911 gold badge66 silver badges1515 bronze badges

0

I'm guessing this was figured some time ago and the correct answer to what was going on was that it was a versions mismatch between local and the on-line flow interpreter.

However, as this question turns up near the top of search results when looking for how to Flow annotate Promises, it's probably worth mentioning that the fix for these types of situations is to specify the Flow type that the Promise resolves to as a Flow angle bracket parameter.

In this case adding a "any" to the Promise annotation silences the error e.g. ...

function process(callback:Function):Promise<any> 
 return new Promise((r,re) => callback)

In the docs over here as well for another example

share|improve this answer

edited Apr 3 at 10:38

answered Mar 28 at 10:25

shufflingbshufflingb

5166 bronze badges

add a comment
 | 

0

I'm guessing this was figured some time ago and the correct answer to what was going on was that it was a versions mismatch between local and the on-line flow interpreter.

However, as this question turns up near the top of search results when looking for how to Flow annotate Promises, it's probably worth mentioning that the fix for these types of situations is to specify the Flow type that the Promise resolves to as a Flow angle bracket parameter.

In this case adding a "any" to the Promise annotation silences the error e.g. ...

function process(callback:Function):Promise<any> 
 return new Promise((r,re) => callback)

In the docs over here as well for another example

share|improve this answer

edited Apr 3 at 10:38

answered Mar 28 at 10:25

shufflingbshufflingb

5166 bronze badges

add a comment
 | 

I'm guessing this was figured some time ago and the correct answer to what was going on was that it was a versions mismatch between local and the on-line flow interpreter.

However, as this question turns up near the top of search results when looking for how to Flow annotate Promises, it's probably worth mentioning that the fix for these types of situations is to specify the Flow type that the Promise resolves to as a Flow angle bracket parameter.

In this case adding a "any" to the Promise annotation silences the error e.g. ...

function process(callback:Function):Promise<any> 
 return new Promise((r,re) => callback)

In the docs over here as well for another example

share|improve this answer

edited Apr 3 at 10:38

answered Mar 28 at 10:25

shufflingbshufflingb

5166 bronze badges

function process(callback:Function):Promise<any> 
 return new Promise((r,re) => callback)

share|improve this answer

edited Apr 3 at 10:38

answered Mar 28 at 10:25

shufflingbshufflingb

5166 bronze badges

answered Mar 28 at 10:25

shufflingbshufflingb

5166 bronze badges

answered Mar 28 at 10:25

shufflingbshufflingb

5166 bronze badges