Functional EDSLs for Web Applications

This thesis aims to make the development of complex web applications easier,faster and safer through the application of strongly typedfunctional programming techniques.Traditional web applications are commonly written in the de facto standardlanguage of the web, JavaScript, which, being untyped, provides no guaranteesregarding the data processed by programs, increasing the burden of testing anddefensive programming.Modern web applications are often highly complex, with multipleinterdependent parts interacting over the Internet.Such applications are traditionally implemented with each component as aseparate program, exposing its functionality to other components throughdifferent API:s over some communication protocol such as HTTP.This process is mostly manual, and thus error-prone and labour intensive,with accidental API incompatibility between components being particularlyproblematic.Even in a conventional typed language, the absence of such incompatibilitiesis not guaranteed.While the different components may well be type-safe in isolation, there is noguarantee that the whole is type-safe as the communication between componentsis not type-checked.We present a web application development framework, based on the Haskellprogramming language, to increase programmer productivity and software qualityby addressing these issues.In our framework, applications with an arbitrary number of components arewritten, compiled and type-checked as a single program, guaranteeing that theapplication as a whole, including network communication, is type-safe.Communication between components is automatically generated by our framework,eliminating the risk of API incompatibilities completely.Additionally, our framework includes astate-of-the-art compiler from Haskell to JavaScript,a novel, lightweight foreign function interface to allow programs to leverageexisting JavaScript code, an embedded language for integrating low-level,high-performance kernels into otherwise high-level web applications, as well asa highly expressive relational database language

Scoping monadic relational database queries

We present a novel method for ensuring that relational database queries in monadic embedded languages are well-scoped, even in the presence of arbitrarily nested joins and aggregates. Demonstrating our method, we present a simplified version of Selda, a monadic relational database query language embedded in Haskell, with full support for nested inner queries. To our knowledge, Selda is the first relational database query language to support fully general inner queries using a monadic interface. In the Haskell community, monads are the de facto standard interface to a wide range of libraries and EDSLs. They are well understood by researchers and practitioners alike, and they enjoy first class support by the standard libraries. Due to the difficulty of ensuring that inner queries are well-scoped, database interfaces in Haskell have previously either been forced to forego the benefits of monadic interfaces, or have had to do without the generality afforded by inner queries