Error On Input Haskell
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Then / Else 1.3 Binding of Type Signatures 1.4 do Notation 1.5 Iterating Over a List 1.6 Guards 1.7 Parentheses 2 See also 1 Common Mistakes and Incorrect Beliefs By Haskell Beginners People going from zero to Haskell are likely to gain a misunderstanding or miss a point haskell parse error on input else that isn't stressed enough. Here are some mistakes that have been observed from multiple sources. 1.1 Indentation haskell parse error on input = Perhaps the first trip-up - you might understand that indentation defines where a code block starts and the lack of an equal amount of
Haskell Parse Error On Input Module
indentation indicates the previous code block ended. What some miss is that then and else, if used within a section of code where indentation matters, must be indented deeper than the if statement. That is, the position of the "else" in
Haskell Parse Error On Input ‘ ’
the following is a syntax error: do if boolean then expr1 else expr2 The following is correct: do if boolean then expr1 else expr2 Or they can be on the same line as the if: if boolean then expr1 else expr2 Fortunately this misfeature is fixed in Haskell 2010, and the first snippet is valid in compilers that support it (e.g. GHC 7.0.1 and later). 1.2 If / Then / Else if-then statements must always include an 'else' portion. It might be best haskell parse error on input ‘where’ not to think of if-then-else as flow control, as in most imperative languages, but think of it as construction of a value using a well formed expression. x = b? y: z; The above is valid C code, a ternary operation, that's used quite commonly as a shortcut to typing out a full if-then-else block. It states that if b is true then x = y otherwise x = z. Notice how this makes no sense without z. Similarly, in Haskell an if/then makes no sense without an else. let x = if b then y -- compare to x = b? y What is x when b is false? One should also recognize that the types returned by the then and else branches must match due to Haskell's strong and static type system. When if is used for sequencing IO it is not uncommon to see an else that returns a null value: main = do startNetwork <- askUser "Network? " if startNetwork then do iface <- initNetworkInterface handlePackets iface else return () Such uses can be more succinct if they use the when function (from the Control.Monad module): main = do startNetwork <- askUser "Network? " when startNetwork $ do iface <- initNetworkInterface handlePackets iface 1.3 Binding of Type Signatures Due to the typeclass overloading of numbers, lists of numbers are often typed as: ghci> [1,2,3,4] :: [Float] This works fine when the list stands alone, but when applying a function take no
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Haskell Parse Error On Input ‘=’
site About Us Learn more about Stack Overflow the company Business Learn more haskell parse error on input if about hiring developers or posting ads with us Stack Overflow Questions Jobs Documentation Tags Users Badges Ask Question x haskell parse error on input import Dismiss Join the Stack Overflow Community Stack Overflow is a community of 4.7 million programmers, just like you, helping each other. Join them; it only takes a minute: Sign up Haskell parser https://wiki.haskell.org/Common_Misunderstandings error in where clause up vote 4 down vote favorite What is wrong with rs definition in first where section? palindrome :: [a] -> [a] palindrome xs = con xs rs where con a b = rev (rev a []) b rs = rev xs -- here where rev [] rs = rs rev (x:xs) rs = rev xs (x:rs) I'm just learning Haskell http://stackoverflow.com/questions/412828/haskell-parser-error-in-where-clause but its syntax rules confuse me. The error message is [1 of 1] Compiling Main ( pelindrome.hs, interpreted ) pelindrome.hs:5:8: parse error on input `rs' syntax haskell share|improve this question edited Apr 24 '12 at 14:26 Riccardo 6,96532362 asked Jan 5 '09 at 11:14 Hynek -Pichi- Vychodil 19.7k33161 add a comment| 2 Answers 2 active oldest votes up vote 12 down vote accepted Your indentation was wrong and i think you can only have one where in there (i could be very well wrong. I'm not a haskell guy). There was also a argument missing for the call to rev (an empty list): palindrome :: [a] -> [a] palindrome xs = con xs rs where con a b = rev (rev a []) b rs = rev xs [] -- here rev [] rs = rs rev (x:xs) rs = rev xs (x:rs) main = print (palindrome "hello") Prints out: "helloolleh" I'm going to try to understand it now. Anyway, have fun! Edit: Makes perfect sense to me now. I think that's the right version. For Haskell indentation rules, read Haskell Indentation share|improve this answer edited Jan 5 '09 at 11:3
place to discuss the implementation and style of computer programs. Moderators: phlip, Moderators General, Prelates Post Reply Print view Search Advanced search 16 posts http://echochamber.me/viewtopic.php?t=33819 • Page 1 of 1 stone915 Posts: 42 Joined: Wed Jan 07, 2009 4:31 am UTC Haskell - "Parse error on input" Quote Postby stone915 » Mon Jan 26, 2009 3:03 am UTC Hi, I'm working on some code to implement a directed graph for a computer science class. The graph will contain a list of ordered pairs made up error on of a vertex and a list of vertices connected to that vertex (an adjacency list). I was given a template with type declarations and function type signatures and was told to implement the functions. I have a couple of the functions written, but I'm encountering some difficulty writing the function to add a vertex with an empty adjacency list to error on input the graph. Whenever I try to compile the code, I get this error message.Code: Select allps2.hs:4:16: parse error on input `insertVertex'
Can anyone tell me what this message means? I've tried removing the if-then-else portion since my prof told us it was optional and using (graph Char Int) instead of graph as a parameter, but neither worked. I also addedCode: Select alltype v = Char
type e = Int
to the code and v,e to the module interface given to us in the template, so if that could be causing the problem please let me know and explain why that's a problem. I'd prefer minimal help, such as an explanation of the error message or a syntax correction rather than a step by step solution, as this is for a class and I'm trying to learn these concepts. Thanks!Code: Select all-- A module for creating and handling directed graphs
module Digraph (AdjList, Digraph, Edge, v, e
insertVertex, insertEdge, makeDigraph, getAdj,
empty, vertex