Partial application supplies only some arguments to a function. Instead of failing, the expression produces a new function that accepts the remaining arguments. For example, `const True` fixes `const`'s first argument and yields a unary function that ignores its input and returns `True`. section 4.2.1
Configure a general function
A line-numbering function can be specialized by fixing its condition arguments: `numberAllLines = numberLines (const True) (const True)`. The result remains a function waiting for the text input. Other fixed conditions create variants that skip numbering or incrementing empty lines. section 4.2.1 section 4.5
Use partial application to derive focused functions from a general implementation rather than repeating its logic. Argument order matters: placing a configuration argument first lets it be fixed early. A generalized `pad` function therefore takes `PadMode` first, allowing `padLeft = pad PadLeft` and `padRight = pad PadRight`. section 4.3.1 section 4.3.2
Listing 4.9 Functions for left and right padding based on a generalized padding function
padLeft :: Int -> String -> String padLeft = pad PadLeft #1 padRight :: Int -> String -> String padRight = pad PadRight #2
Fixing the padding mode leaves a reusable function that still accepts width and string arguments. section 4.3.2
Partial application also shortens function arguments passed to higher-order functions. `map (maybe "" show) numbers` fixes the first two arguments of `maybe`, leaving a function that converts each `Maybe Int` to a string. Eta reduction can remove an explicitly named parameter when a definition merely passes it on, making equivalent definitions more concise. section 4.3.4 section 4.2.1
