For bringing your product to market, there isn't just one metric for success. That depends on your business needs. Haskell is the pure functional programming language that brings tangible benefits over its competitors for a variety of TTM priorities. Let's explore four of them.
You may want to bring your product to market as quickly as possible, because you have direct competitors; you have to produce a functioning demo for your investors; or your success in the market is time sensitive. Haskell speeds up this process in various ways.
Constuct things correctly the first time: Haskell is statically typed, which is a form of program verification that guarantees correctness of certain properties, like in Java or C#. Unlike Java or C#, Haskell is a pure functional language, leading to verification of far more portions of the program source code. With feedback from the compiler while developing, your developers are guaranteed a certain level of correctness and this allows them to concentrate on your domain business logic. As written elsewhere, worst practices should be hard. Also, a case study on Haskell vs C# for contract writing.
Reduce testing time: Haskell emphasizes the correct by construction approach, which is to use the type system to verify that program parts are combined in only the ways that will not crash and that make sense. Examples range from the simple to advanced. For example, Haskell web frameworks like Yesod prevent XSS and accidental broken links statically. The more surface area of your problem covered by static analysis, the less time and effort is needed by your developers for writing unit and integration tests, and the tests blow up less often in continuous integration.
Make builds reproducible: Haskell projects that are built with the Stack build tool, are guaranteed reproducible builds using a stable set of packages, with Stack also providing docker support out of the box, adding an additional layer of reproducibility. If a reproducible build is created on one developer's machine, it will work on any. This significantly reduces ramp up time for your developers, and makes continuous integration trivial for your devops people.
Use the concurrency: Many problems are solved more easily with concurrency (networking, file I/O, video/image/document processing, database access, etc.) simply because the programming model is easier to understand. Haskell has some of the best concurrency support of any popular language, it has a breadth of tools, efficiency, stability, and it is trivial to use, out of the box. Let your developers use it. See Beautiful concurrency for more about concurrency in Haskell. Additionally, the code doesn't have to be rewritten in an arcane style like in NodeJS to gain concurrency.
You may not need to ship as soon as possible, but to ship on schedule, for a demo, a conference or as promised to your investors or customers. For this, there is another mitigating feature of Haskell.
Types shed light on scope: Using the type system of Haskell to model your domain logic helps to expand the "fog of war" that we experience with project scope: there are many unknowns, and we need to enumerate all the cases and kinds of things. Armed with this UML-without-the-UML, you can have confidence in how much scope you can cover now for the current shipping schedule, and what needs to come in version 2. This gives confidence in time estimates made by your developers. See Haskell-Providing Peace of Mind for a case-study. See Using Haskell at SQream Technologies for a technical demonstration.
Avoid build system detours: Reproducible builds help to avoid the inevitable build system detours that happen on large projects, where developers are wasting time getting the build system to work on eachother's machines and debugging failures. Stack is reproducible out of the box.
You might want or need to be cost-effective in your development cycle, using as few developers or machines as possible. Haskell also reduces development costs.
Less testing is required: Haskell software requires less testing. Your developers always write tests, but with Haskell they can write fewer, and spend less time on it. Furthermore, fewer developers are needed to achieve a stable system, because a type system helps limit scope of possibilities, and lets your developers manage complexity.
Use Haskell's raw performance: Additionally, Haskell is a fast language on a single core. It is compiled to native machine code, and is competitive with C# and Java. At times it is competitive with C, see Haskell from C: Where are the for loops? for a technical demonstration. This means that you need fewer machine resources, and fewer machines. Haskell is also easy to write concurrent code for, which means you can make use of those additional cores on your machines.
Most developments put a high priority in flexibility to change, or should. But you may have particularly pronounced need for flexibility in changes to requirements without disruption. This is perhaps Haskell's strongest, most desirable advantage.
Correct by construction extends to reconstruction: Making a change to a well-typed system in Haskell is far more stable and reliable than its competitors because correctness invariants are maintained with any change of the code. See also this case study by Silk.
Less maintenance of the test suite under change: This requires less maintenance of the test suite, because static analysis gives your developers immediate feedback at a granular level, whereas a test suite typically does not guide them through the change process, it only tells them what their out-of-date specification expects. Once the change process is complete, updating the test suite becomes easier.
Types are rarely buggy: It's very rare to design a data type that has bugs in it, because they are so simple. Meanwhile a unit test or integration test suite presents additional developer overhead because it itself is a program that requires maintenance too.
You may find that your project requires hiring new developers and building a team. Now is the perfect time to hire Haskell developers. Like Python developers 10 years ago, Haskell developers are self-selecting; they learn it because it's a better language, not because it will guarantee them employment. At the same time, at this stage in Haskell's development, the wealth of practical, stable packages available indicate an infusion of pragmatic, experienced programmers.
In summary we've seen that: