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NonEmptyList map gives me :: Seq<'b>

See original GitHub issue

given a NonEmptyList<'a> and running that thru FSharpPlus.Operators.map I am getting a Seq<'b> where as I expected it to return a NonEmptyList<'b>. What am I doing wrong? Only when I fully qualify the call to map like

NonEmptyList.map transform someNonEmptyList

the result is the correct type of functor

Issue Analytics

State:
Created 7 years ago
Comments:7 (4 by maintainers)

Top GitHub Comments

1reaction

gustycommented, Jan 21, 2017

@mausch the instance is within the class. Although now that FsControl was merged, we can put it in the Invokable, I have the impression that is good to keep the separation and only specify instances for primitive types in the Invokables.

@robkuz I think this issue was recently solved here but probably you are using the latest nuget which is previous to that fix.

The problem is that in some cases interfaces “eat” all instances before the instance declared in the type. This happens because F# does some automatically conversions (upcasts) in some situations.

So I did some workarounds to give either less priority to the interfaces overload, in cases where it’s still usable, or to reject compiling it unless it’s explicitly upcasted as an interface, which is the case of the functor for NonEmptyList.

Try the latest code and let me know if it works in your scenario. If so I will see if I can upload a new nuget tonight.

0reactions

robkuzcommented, Feb 20, 2017

Seems to work!