Is there a way in GHCI to show a fully parenthesized version of a statement? I've found myself wanting to do that sometimes to help me understand a piece of code that I'm not familiar with. Sometimes the conciseness of the masters obscure things for us n00bs, and anything to help me break these beasts apart seems to help. ...
I am trying to walk through the functions in Data.List of the Haskell stardard library and get an error when trying "permutations". What am I missing here? Thanks. Prelude> map (\b-> b*b) [1,2,3] [1,4,9] Prelude> permutations "abc" <interactive>:1:0: Not in scope: `permutations' ...
Hi. I am new to Haskell, using Ghci. I have a function, called three, that I want to write as let three = \x->(\y->(x(x(x y)))) OK, this works, but when I try three (2+) 4 It does not work. Instead, I get some "cannot construct infinite type" error. Please help me. ...
One way to calculate 2^8 in haskell is by writing product(replicate 8 2) When trying to create a function for this, defined as follows... power1 :: Integer → Integer → Integer power1 n k | k < 0 = error errorText power1 n 0 = 1 power1 n k = product(replicate k n) i get the following error: Couldn't match expected type 'Int' again...
There are differences between Hugs, Yhc and GHCi? If there are differences, What are they? ...
I'm using ghci 6.10.4 at the dos command line in XP, and also in emacs using haskell-mode-2.4 When running programs that operate on stdin, is there a way I can redirect a file to be stdin? For example if I have a function called main that reads from stdin, I can't do: *Main> main < words.txt Is there another way? Also I would like t...
I get the error "Not in scope: x" when doing as follows... blanks :: Sudoku -> [Pos] blanks (Sudoku su) = [ fst x | x <- posSud | isBlank (snd x) ] where isBlank Nothing = True isBlank _ = False posSud = zip ixPos (concat su) ixPos = zip ixRows ixCols ixCols = concat (replicate 9 [0..8]) ixRows ...
I am trying to write a prop that changes a Sudoku and then checks if it's still valid. However, I am not sure how to use the "oneof"-function properly. Can you give me some hints, please? prop_candidates :: Sudoku -> Bool prop_candidates su = isSudoku newSu && isOkay newSu where newSu = update su aBlank aCandidate aCandi...
I've been going through a Haskell tutorial recently and noticed this behaviour when trying some simple Haskell expressions in the interactive ghci shell: Prelude> 1.1 + 1.1 == 2.2 True Prelude> 1.1 + 1.1 + 1.1 == 3.3 False Prelude> 1.1 + 1.1 + 1.1 > 3.3 True Prelude> 1.1 + 1.1 + 1.1 3.3000000000000003 Does anybody know why that is? ...
Is it possible to query the ghci for the type it inferred for a function inside another function? ...
Hi I've defined a Tree data type in Haskell and an associated 'size' method which calculates the number of elements in the tree. This worked before, however I have updated the Tree data type to use named fields as in the following definition: data Tree a = Empty | Leaf {value::a} | Node {left :: (Tree a), value :: a, right :: (Tree a...
I'm trying to compile this function from Learn You a Haskell for Great Good. removeNonUppercase st = [ c | c <- st, c `elem` ['A'..'Z']] by placing it into a removeNonUpperCase.hs file. It compiles fine, but when passing the argument: ghci> removeNonUppercase "Hahaha! Ahahaha!" the compiler says: <interactive>:1:0: Not in ...
I'm trying to define any simple function that spans multiple lines in ghci, take the following as an example: let abs n | n >= 0 = n | otherwise = -n So far I've tried pressing Enter after the first line: Prelude> let abs n | n >= 0 = n Prelude> | otherwise = -n <interactive>:1:0: parse error on input `|' I've a...
In test.hs, I have: doubleMe x = x + x In ghci, I type: Prelude> :l test [1 of 1] Compiling Main ( test.hs, interpreted ) Ok, modules loaded: Main. *Main> doubleMe 9 <interactive>:1:0: Not in scope: `doubleMe' *Main> Why? How to fix? ...
Intro: While checking out snoyman's "persistent" library I found myself wanting ghci's (or another tool) assistance in figuring out stuff. ghci's :info doesn't seem to work as nicely with type-families and data-families as it does with "plain" types: > :info Maybe data Maybe a = Nothing | Just a -- Defined in Data.Maybe ... > :inf...
I'm playing around with GHCi for the first time, and I'm having some trouble writing multi-line functions. My code is as follows: Prelude> :{ Prelude| let diffSquares lst = abs $ squareOfSums lst - sumOfSquares lst Prelude| where Prelude| squareOfSums lst = (fst (sumsAndSquares lst))^2 Prelude| sumOfSquares lst = snd (sumsAndS...
How to I define the equivalent of this function (taken from learnyouahaskell) inside GHCi? import Data.List numUniques :: (Eq a) => [a] -> Int numUniques = length . nub Without the type declaration, GHCi accepts the function definition, but it ends up with an unhelpful type: Prelude Data.List> import Data.List Prelude Data.Li...
I've installed the such-and-such a package using cabal, and I can build a program that depends on it using cabal build. But when I load the same program in ghci, ghci complains that it "Could not find module `such-and-such'". I'm surprised this doesn't "just work." How do I tell ghci where to find the packages I've installed with cabal?...
I get an error in ghci when I try to define a new type: Prelude> data Point = Pt Int Int <interactive>:1:0: parse error on input `data' Prelude> let data Point = Pt Int Int <interactive>:1:4: parse error on input `data' What am I doing wrong? ...
I have the following function defined: ex 1 x = 1 ex 0 x = 0 ex b x = b ** x Then, when I execute the following: 1 `ex` (sum [1..]) it tries to calculate the sum of the infinite sequence, instead of being lazy and returning 1. Why? EDIT: Upon further investigation, I found that laziness happens if I define the ex function in a ...