Create persistent `Map` populated from `main`

[atravers]

@klequis

From main I’m reading in a file and turning the data into [(Int, RobotPart)]. I want to create a Map using Map.fromList and have it persist for the life of the program and have it accessible to other functions that take user input such as getPart 1 and compareParts 1 2.

[…]

partsDB :: Map.Map Int RobotPart  -- no pesky IO!
                 ⋮

…so in other words, you’re trying to disguise an inherently-mutable data source - a file in this case - as a regular (immutable) Haskell value:

Using unsafePerformIO safely?

A colleague of mine Victor Miraldo asked me today whether I know how to use unsafePerformIO safely. And I realized I have no idea. Do you? Or do you know of an existing blog post / other resource that addresses this? Context: Victor is writing a program that needs to call out to an SMT solver. An SMT solver acts like a pure function: it gets information in and then deterministically produces a result based (only) on its input. But because the SMT solver is invoked via a separate process, the in…