• Alexis King's article Parse, Don't Validate

the difference between validation and parsing lies almost entirely in how information is preserved

• Shotgun parsing (original academic paper)
• Mixing processing and validating data

Shotgun 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 failure

• Conditionally return types
• Don't have to repeatedly check condition
• Look out for "lowest common denominator" built-in values being passed around (like empty String)
• Maybe.withDefault might indicate an opportunity to parse

Two ways to use this technique:

• Weaken return type

• Strengthen input type

• Design by contract

• Elm Radio JSON decoders episode