HaskRel: HaskRel, Haskell as a DBMS with support for the relational algebra
HaskRel aims to define those elements of the relational theory of database management that Haskell can accommodate, thus enabling Haskell (or more precisely GHC) in its own right as a DBMS with first-class support for those parts of the relational model. It does not qualify as a proper RDBMS since it as-is only defines the relational algebra, relational variables and relational assignment. It does not define the relational calculus, views, constraints and transactions (beyond the fundamental requirement that the tuples of relations are to be unique), certain operators like relation valued aggregate operators, nor a few minor or even deprecated operators such as DIVIDE. The implemented parts are decently complete even if there are major implementation shortcomings that prevent this from being practically usable as an actual DBMS.
I refer to it as "first-class" since the types of the relational model are first-class types to Haskell, and the Haskell type system is able to induce the type resulting of relational expressions (for instance that a natural join of two relations results in a relation with a heading that is the setwise union of the headings of the original relations).
The examples in this documentation are based on "the old warhorse" that is the
suppliers-parts database (see  for more). This gives a body of relational
expressions with known results to base examples upon. See also
examples/SuppliersPartsExamples.hs (not visible from this documentation) for
Haskell versions of a selection of the Tutorial D expressions given as
examples in chapters 6 and 7 of . These can be run by starting
examples/suppliersPartsDB.sh and then running
snrt2ndExamples. While most
Tutorial D expressions translate fairly verbatim to Haskell there are a few
where one must be a bit more explicit. While most Tutorial D expressions
translate fairly verbatim to Haskell there are a few where Haskell is stricter
than Tutorial D and one must be a bit more explicit.
$ is always used after
rPrintTyped in the examples to
keep them uniform (and so it kinda looks like a prompt), even when not
required. The short forms
pt are used whenever there isn't a
conflict with other identifiers, whereas for the SuppliersPartsExample, which
has a relvar "
rPrint is used instead of
p for presentation of
relational objects without type information.
Since this builds on both Haskell and relational theory this documentation uses terms as they have been established in material related to either. Several terms of Haskell and HList have been redefined in terms of relational theory in this library, mostly to illustrate how terms and concepts have been mapped from the latter to the former. (I'm trying to keep this open to change later if it turns out to be an unhelpful crutch.)
The following table gives a quick overview of either terms or concepts as found in Haskell, the relational model (as presented in ), HaskRel and SQL, and how they are mapped from the second to the first:
┌───────────────────────────┬────────────────────┬────────────┬────────────────────────────────────────────────┐ │ haskell │ relModel │ haskRel │ sql │ ╞═══════════════════════════╪════════════════════╪════════════╪════════════════════════════════════════════════╡ │ Data.Tagged.Tagged │ attribute │ Attr │ field, column │ │ Data.HList.Record.Record │ tuple │ RTuple │ row │ │ ( Set (Record a) ) │ relation │ Relation a │ table │ │ FilePath (Set (Record a)) │ relvar │ Relvar a │ table │ │ Data.HList.Record.Label │ attribute name │ Label │ field name, column name │ │ Data.HList.Record.Labels │ attribute name set │ Labels │ list of field/column names │ │ function, operator │ operator │ function │ operator, function, procedure, routine, method │ └───────────────────────────┴────────────────────┴────────────┴────────────────────────────────────────────────┘
Found in "example/Terminology.hs". Note that this is just an overview, study of  or , Haskell itself, HList and HaskRel is required to see how terms and concepts correlate.
The term "RTuple", or "r-tuple", is chosen to simultaneously distinguish the concept from Haskell tuples while relating it to tuples of the relational model. For the type of either "Record a" or "Set ( Record a )" in Haskell the term "heading" is used in relational theory, and "row type" or "composite type" in SQL. In relational theory the term "scalar" is used to refer to data types that are neither tuples nor relations, which corresponds to everything but "Record a" or "Set ( Record a )" in Haskell. Note also that HaskRel does use the term "table", but then only in the sense of "presentation of a relation value" (see above).
The HaskRel library
Not all modules of this library are relevant to gain an understanding of how it functions, the next part to go to at this point is Database.HaskRel.RDBMS, and the modules it reexports.
- SQL and Relational Theory, 2nd ed. (2011), C.J. Date
- The Third Manifesto, C. J. Date and Hugh Darwen, February 7th, 2013
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|Versions [faq]||0.1.0.0, 0.1.0.1, 0.1.0.2|
|Dependencies||base (==4.8.*), containers (==0.5.*), directory (==1.2.*), ghc-prim (==0.4.*), HList (==0.4.*), tagged (==0.8.*) [details]|
|Copyright||Thor Michael Støre 2015|
|Author||Thor Michael Støre|
|Source repo||head: git clone https://github.com/thormick/HaskRel|
|Uploaded||by thormick at 2015-11-16T23:52:25Z|
|Downloads||2337 total (12 in the last 30 days)|
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