Contribution:

Reusable abstractions for accessing company data

Characteristics

Feature:Total is an example of an operation on company salaries. Other options on company salaries are conceivable, too; see, for example, Feature:Median. Feature:Cut is an example of an operation for transforming companies in the salary position. Other options on company salaries are conceivable, too; see, for example, Feature:Cut. It is quite common to set up reusable abstractions (potentially higher-order functions) to process heterogeneous data structures in all kinds of ways. That is, we generalize over cutting salaries by setting up a transformation function which is parameterized in the function to be applied in salary positions and we generalizing over totaliong salaries by setting up a function to extract all salaries as a list.

Illustration

We total salaries by leveraging the extraction of salaries from companies:

```total :: Company -> Float
total = sum . getSalariesFromCompany
```

Computing the median salary can rely on the same function for salary extraction:

```median :: Company -> Float
median c = sort ss!!(length ss `div` 2)
where
ss = getSalariesFromCompany c
```

We cut salaries by leveraging the function for transforming salaries in companies:

```cut :: Company -> Company
cut = transformSalariesInCompany (/2)
```

Raising salaries can rely on the same function for transforming salaries in companies:

```raise :: Company -> Company
raise = transformSalariesInCompany (*1.01)
```

Architecture

Salary extraction

```getSalariesFromCompany :: Company -> [Salary]
getSalariesFromCompany (Company n ds) = concat ds'
where
ds' = map fromD ds
fromD (Department n m ds es) = m' : concat ds' ++ es'
where
m' = fromE m
ds' = map fromD ds
es' = map fromE es
fromE (Employee _ _ s) = s
```

Salary transformation

```transformSalariesInCompany :: (Salary -> Salary) -> Company -> Company
transformSalariesInCompany f (Company n ds) = Company n ds'
where
ds' = map inD ds
inD (Department n m ds es) = Department n m' ds' es'
where
m' = inE m
ds' = map inD ds
es' = map inE es
inE (Employee n a s) = Employee n a (f s)
```

Discussion

Heterogeneous data structures such as companies breaking down into departments, employees, names, addresses, and salaries. We could set up transoformation and extraction helpers for other ingredients of compoanies. See Contribution:haskellFunctorial for an alternative approach of organizing access to positions of a certain type. Ultimately, we could leverage Theme:Haskell genericity to perform traversals on heterogeneous data structures; see, for example, Contribution:haskellSyb.

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Ralf Lämmel edited this article at Sun, 09 Jul 2023 17:53:21 +0200

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