Using fp-ts Array's isNonEmpty with fp-ts-std Predicate's when

[ammarriq]

Can somebody please help me to use fp-ts-std when with fp-ts Array's isNonEmpty?

I'm currently using it like this;

import { when } from "fp-ts-std/Predicate"
import { isNonEmpty } from "fp-ts/Array"

when(isNonEmpty([]), do something)

but it is displaying an error;
Cannot assign boolean to predicate

[samhh]

I think you mean the when in fp-ts-std/Function.

isNonEmpty([]) isn't a predicate, it's a boolean. Remove the ([]) application and that part will typecheck. You'll also need to shift the do something as everything in fp-ts-std is curried.

Example:

import * as A from 'fp-ts/Array'
import { when } from 'fp-ts-std/Function'

const f: Endomorphism<string> = when(A.isNonEmpty)(A.append('another item for my non empty array'))

NB as currently written when won't carry through the refinement of isNonEmpty.

[ammarriq]

this is what I am doing but this also throws error:

const mapping = map((body) => ({ value: body.id, label: body.classname }))
const classOption = when(isNonEmpty)(mapping)(classesQuery.data ?? [])

Argument of type 'IClass[]' is not assignable to parameter of type '{ value: any; label: any; }[]'.

[samhh]

Add an explicit type annotation to mapping.

[ammarriq]

const mapping: (body: IClass[]) => ISelect[] = map((body) => ({ value: body.id, label: body.classname }))
const classOption = when(isNonEmpty)(mapping)(classesQuery.data ?? [])

now mapping assigned to when, says;
Argument of type '(body: IClass[]) => ISelect[]' is not assignable to parameter of type 'Endomorphism<IClass[]>'

btw thank you for your help

[samhh]

np 🙂

when needs the mapping function to be an endomorphism i.e. same input and output type. That way the result is the same regardless of whether or not the condition matches (whether or not the function is applied).