ghci

When I compile a Haskell file with ghci , typically with :load , and if there is no type error, all the expressions are loaded in the ghc interpreter. It's very nice: I can play around with :t to figure out the type of various expressions. My problem is: if there is a tiny error somewhere, ghci is not able to load anything (not even the imported modules!!), which makes finding the right types even more difficult. I always do the same: comment out all the bits that do not typecheck, find the relevant types wiht :t in ghci, and de-comment. But this is so tedious! Is there a better workflow for

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? titaniumdecoy , I remember being helped with this sort of GHCi mystery when I learned the frequently made point that writing things like 'let square x = x * x' inside the GHCi is like writing let a = f b with do notation in the IO monad -- say in this sort of example: palindromize :: IO () palindromize = do a <- readFile "foo.txt" let b = reverse a

I am not able to get this line of code compiled in Haskell but it works on my professor's system. I use ghci version 7.6.2. data Eq a => Shape a = Shape a More precisely, this is the error I am getting [1 of 1] Compiling Main ( test.hs, interpreted ) test.hs:1:6: Illegal datatype context (use -XDatatypeContexts): Eq a => Failed, modules loaded: none. What is the mistake here? Thanks Your professor is probably using an older version of GHC. The line you posted uses a feature which has quite recently been removed. The possible solutions are: Remove Eq a => and write data Shape a = Shape a . As

I am writing a module that relies on optimization. I want to test this module in ghci. But starting ghc in --interactive mode automatically disables optimization; if I compile the module with -O and then try to load it in an interactive session, ghc insists on loading it in interpreted mode. For a simple test case to distinguish optimized and unoptimized modules, isOptimized below evaluates to True with optimization on, but False with optimization off: isOptimized :: Bool isOptimized = g g :: Bool g = False {-# NOINLINE g #-} {-# RULES "g/True" g = True #-} Either use ghci -fobject-code -O

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.List> let numUniques' = length . nub Prelude Data.List> :t numUniques' numUniques' :: [()] -> Int The resulting function only accepts a list of units as a parameter. Is there a way provide type declarations in GHCi? Or is there another way to define functions like these

When I use GHCi, I almost always end up importing Control.Applicative , Data.List , etc. . Is there a way to configure GHCi to automatically import those modules. Also, after importing them, how do I keep the prompt from being insanely long? Prelude Control.Applicative Data.List Database.HDBC Database.HDBC.Sqlite3 System.Directory> GHCi looks for its configuration file at ~/.ghc/ghci.conf on Unix-like systems. %APPDATA%\ghc\ghci.conf on Windows. The configuration file syntax is simple: it's a list of GHCi commands to execute on startup. For example, your ghci.conf could contain: import Control

I'm trying to enable XRankNTypes in GHCi. How do I do this? :set -XRankNTypes . The -X part is a compiler switch, by the way, extension is called just RankNTypes . 来源： https://stackoverflow.com/questions/12584884/how-do-i-enable-language-extensions-from-within-ghci