Master programming syntax, solve interactive challenges & run live code in Syntaxis Explore Arena →
menu_book Course Curriculum (Haskell) expand_more
Values and Types
Top-level bindings Type signatures Int and Integer Float and Double Char and String Bool values Tuples Lists Maybe values Either values
Functions
Function application Curried functions Partial application Infix functions Lambda expressions Function composition Point-free style Guards Where bindings Let expressions
Pattern Matching
Literal patterns Tuple patterns List cons patterns As-patterns Wildcard patterns Case expressions Pattern guards Irrefutable patterns Newtype patterns Record patterns
Lists and Higher Order
Map Filter Foldl Foldr Zip List comprehensions Take and drop Scanl Any and all ConcatMap
Algebraic Data Types
Data declarations Record syntax Recursive data types Newtype Type synonyms Parameterized types Deriving Show Deriving Eq Enum types Smart constructors
Type Classes
Eq constraints Ord constraints Show instances Custom type classes Instance declarations Functor Applicative Monad Semigroup Monoid
IO and Effects
Main IO action Pure and IO boundary GetLine PutStrLn ReadFile WriteFile WithFile Sequence actions Traverse in IO Exception handling
Advanced and Tooling
Lazy evaluation Strict evaluation with seq Bang patterns State monad Reader monad Writer monad STM transactions MVar synchronization QuickCheck properties GHC language pragmas
Haskell / Advanced and Tooling

Reader monad

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

12 min read • Advanced

Reader monad 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 Reader monad 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-0075-bbb614175c.hs
haskell Try It Live 
safeHalf :: Int -> Maybe Int
safeHalf n = if even n then Just (n `div` 2) else Nothing
main = print (safeHalf 20 >>= safeHalf)
-- Independent check: any (>5) [1,7,3]
Edge case / engineering note

For Reader monad, 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

Lesson quizzes

Reader monad Mastery Check →
← PREVIOUS
State monad
NEXT →
Writer monad