时间线 · 2026
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Section 9.1 Monadic Classes
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We have actually been using the derived Show instances all along for printing out trees and other structures we have defined. The code in the tutorial for the Eq and Ord instance of Tree is created implicitly by the deriving clause so there is no need to write it here.
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No examples are provided for 5.1 or 5.2. The standard Prelude contains many instance declarations which illustrate the Eq, Ord, and Enum classes.
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import IO
Section 7.3
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import IO (isEOFError)
Both putStr and getLine are actually in the prelude.
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Section 6.1
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Names in the basic class structure of Haskell cannot be hidden (they are in PreludeCore) so we have to modify the names used in the tutorial.
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The function take using a case statement instead of multiple equations:
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Now for details of pattern matching. We use [Int] instead of [a] since the only value of type [a] is [].
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Observing lazy evaluation can present difficulties. The essential question is does an expression get evaluated?. While in theory using a non-terminating computation is the way evaluation issues are examined, we need a more practical approach. In expressions, Haskell uses error to denote bottom. Evaluation of error will halt execution and print the attached error message. The error function can be used to create stub functions for program components which have not been written yet or as a value to insert into data structures where a data value is required but should never be used.
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淚腺點會失調
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Haskell has both identifiers, like x, and operators, like +. These are just two different types of syntax for variables. However, operators are by default used in infix notation.
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Section: 3 Functions
第 3 节 函数
add :: Int -> Int -> Int
add x y = x+y
e1 :: Int
e1 = add 1 2
This Int -> Int is the latter part of the signature of add:
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Warning: brackets in Haskell are used in three different sorts of expressions: lists, as in [a,b,c], sequences (distinguished by the ..), as in [1..2], and list comprehensions (distinguished by the bar: |), as in [x+1 | x <- xs, x > 1].
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Since type synonyms are part of the type language only, it's hard to write a program which shows what they do. Essentially, they are like macros for the type language. They can be used interchangeably with their definition:
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data Tree a = Leaf a | Branch (Tree a) (Tree a) deriving Show
The following typings are implied by this declaration. As before, this is not valid Haskell syntax.
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人生到處知何似
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The type Bool is already defined in the Prelude so there is no need to define it here.
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Start with some sample lists to use in test cases:
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Introduction
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荒徑饒松子
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Kubernetes 节点驱逐
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如露亦如電
