fp-ts | Functional programming in TypeScript | Functional Programming library

I'm trying to create a simple helper function using fp-ts/Option and Typescript that takes a an arbitrary value and returns it as an Option<T>.

The problem I'm having is, that no matter what I do, it includes undefined in the option. In somefile.tsx I have an object whose key value uses the toOption function. I need for that value to be an Option only but instead it reads the type as (property) airDate: Option<(string & Date) | undefined>

Using type-guards

Ternary

Conditional

They all yield the same type in the end: (property) airDate: Option<(string & Date) | undefined>

I am working on a project that using fp-ts

I have 2 TaskEither object like TaskEither<ErrorA, A>, TaskEither<ErrorB, B>

I wanted to merging this objects contents and create new TaskEither<ErrorA, A>,
Example A object = {a: 123, b: 456, c: 0}

Example B object = {c: 789}

I wanted to create newObject: TaskEither<ErrorA, A>

and If everything goes well, expecting value should be {a: 123, b: 456, c: 789 }

I try to replicate this code from this blog, but running into some pretty obscure errors

Playground Link

I am implementing a pipe function but with one difference from regular implementations: rather than taking functions as arguments, it must accept an array of functions passed as a single argument.

Like most implementations, my pipe implementation captures the input and output types of each function using type varaibles.

For comparison, common implementations like https://gcanti.github.io/fp-ts/modules/function.ts.html#pipe would look like this:

Although both implementations work similarly, the former is less capable at infering the input and output types variables – specifically when those types are inferred by generic functions within the array itself.

We can help the compiler to infer A and B:

However, it is desired that the user of pipeWithArr can: a) be able to pass it generic functions b) not need to provide explicit types

Are there any workarounds to make pipeWithArr work like pipeWithArgs or is this a limitation of the compiler? Or, a misunderstanding of how it should work?

I am using fp-ts and in this case, Options. Lets say I have an example type:

and a record of these types:

I have discovered the same "theme" several times: I want to write a generic function that can, given a property name and the record type (which can be ExampleType or any other type with at least an option property), turns this:

into this:

Notice I give it the b key and it filters all records that have no value for b, while extracting the value from the option in case it exists and returning it.

However, I have no idea how to type such a function f without losing the type information. The return type should be something like this:

But I fail to figure out how to infer this dynamically if it is even possible. I mean, the b: number comes from the fact that b is an O.Option in T, but as b is dynamic, that makes it harder to actually infer anything.

Does anyone have any ideas?

I am using arg and fp-ts. I want to have a function that returns type based on one of the inputs key value.

But it says that I can't index with K in keyof T, despite K being the actual keys. And it won't see K in Option. I need some help with types.

Result

I am new to fp-ts. Let's say I have a function (path: string) => TaskEither<Erorr, T> which reads and parses config, and I want to write a test for that.

So far I have:

But it fails due to timeout. And also I am unsure if I implemented fold correctly. How do fold TaskEither to Task in general and then call it asynchronously?

I am new to fp-ts. I am used to scala for comprehensions when dealing with combining Either, IO and ... The question is, let's say we have function with signatures

And I want to combine them, and I get something along the lines of

Which should be flattened.

How to combine function of types T -> M<T> where M is IO or Task, and T -> M<Error, T> where M is Either, TaskEither or IOEither, flattening in process, like with scala for comprehensions?

I want to do what I think is the simplest thing, map through a plain old Javascript object, using fp-ts constructs.

There are a lot of operators I would like to use — map, filterMap, and so on — but they seem to require the Map type (which I assume is the Javascript Map object) — and there does not seem to be any easy way to convert back and forth between Map and Javascript object, which in Typescript is the standard representation of algebraic data types and of its Record<K,V> type.

This seems like a pretty big hole. Please don’t tell me I have to switch to lodash...

In the example above since Article requires 2 params that are side-effects. The question is on createArticle and if that could be written as a point free function.