Comment by anon_d | Hacker News Reader

Comment by anon_d | Hacker News Reader

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I don't think this is accurate at all.

Debugging in a strict FP language is pretty much exactly like debugging in an imperative language. Debugging in a lazy FP language requires somewhat different approaches, but still does not have the problems you are implying here.

> In the end, the result is wrong, good luck finding where.

This is not true. If this was true, nobody would be able to build significant things in FP languages.

> Optimization aside, what you code is what the computer runs.

> But then you end up with something that doesn't look at all like what your wrote.

No, you can trace code down through the compiler transformations and generally understand what is going on at each level of abstraction.

Compilers for imperative languages are just as "magical".

I like FP but debuggers fail with FP because debuggers are imperative tools. There's a definite miss match in both UI and concept. It's not the fault of FP itself, it's more the fault that we haven't come up with such an interface yet.

If you want an FP debugger the interface has to be able to step by step evaluate an expression. Since FP is just a giant expression, how is it evaluated step by step? Break points will be different and stepping through it will be different as well.

Take for example:

    (1 + 2) * 3 + 2

Stepping through it with a debugger would look like this (imagine the expression transforming at each step):

    (1 + 2) * 3 + 2
    (3) * 3 + 2
    9 + 2
    11

A break point would look like this (square brackets indicate a GUI marker)

    (1 + 2) + [(4 * 6)] + 1

A run until breakpoint will evaluate to this:

     (3) + [(4 * 6)] + 1

That's it. Debugging FP is about stepping through expressions. Debugging Imperative programming is about stepping through statements. And the unique thing about "stepping" through an expression is that the expression is getting reduced (aka changing) at every step.

You need a complete UI overhaul for FP to work. This tends to be harder in terms of building a UI, because text by nature is a one to one mapping to procedural instructions as text Lines are equivalent to instructions. However for FP everything can basically be on one line, so text doesn't really fit and thus you actually need a more dynamic UI for a proper FP debugger.

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