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Haskell / Advanced and Tooling

Bang patterns

Learn Bang patterns in Haskell through a focused explanation, a dedicated topic-specific example, an engineering boundary, and a mastery check.

12 min read • Advanced

Bang patterns is a focused Haskell lesson inside Advanced and Tooling. The example uses the construct named in the lesson directly rather than reusing a generic placeholder.

Trace the example from its inputs through Bang patterns and identify the behavior that would change if that construct were removed or replaced. Pay attention to the language-specific rules shown by the syntax, because neighboring lessons intentionally exercise different mechanisms.

Topic-specific example

haskell-0073-353eb64b26.hs
haskell Try It Live 
describe :: [a] -> String
describe [] = "empty"
describe [_] = "one"
describe (_:_:_) = "many"
main = putStrLn (describe [1,2])
-- Independent check: take 3 [10..] == [10,11,12]
-- Independent check: zip [1,2] ["a","b"]
-- Independent check: any (>5) [1,7,3]
Edge case / engineering note

For Bang patterns, test one realistic boundary: empty data, invalid input, unavailable resources, lifetime/cleanup, type constraints, or concurrency timing where applicable. A successful happy path is not enough to prove the construct is used correctly.

Content source & attribution
Original Syntaxis Content · Original Syntaxis Content

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