011: Parse, Don't Validate

Alexis King's article Parse, Don't Validatethe difference between validation and parsing lies almost entirely in how information is preservedShotgun parsing (original academic paper)Mixing processing and validating dataShotgun parsing is a programming antipattern whereby parsing and input-validating code is mixed with and spread across processing code—throwing a cloud of checks at the input, and hoping, without any systematic justification, that one or another would catch all the “bad” cases.Why the term "parse"?a parser is just a function that consumes less-structured input and produces more-structured output [...] some values in the domain do not correspond to any value in the range—so all parsers must have some notion of failureConditionally return typesDon't have to repeatedly check conditionLook out for "lowest common denominator" built-in values being passed around (like empty String)Maybe.withDefault might indicate an opportunity to parseTwo ways to use this technique:Weaken return typeStrengthen input typeDesign by contractElm Radio JSON decoders episode