ghci

I was thinking it would be nice to set up my global GHCI config such that my commonly-used imports occur automatically when the packages that provide them are present. I tried adding this to ~/.ghc/ghci.conf : :set -XCPP #ifdef MIN_VERSION_containers import Data.Set (Set) import qualified Data.Set as Set import Data.Map (Map) import qualified Data.Map as Map #endif But apparently that does not work. > stack repl Configuring GHCi with the following packages: GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help <interactive>:24:1: error: parse error on input ‘#’ <interactive>:29:1: error

I am a novice in Haskell. Here's some simple code: module Src ( -- The 'Answer' type isn't exported Shape(Circle), -- i.e 'Rectangle' data constructor isn't exported Point(..), area, nudge ) where data Answer = Yes | No deriving (Show) data Point = Point Float Float deriving (Show) data Shape = Circle Point Float | Rectangle Point Point deriving (Show) area :: Shape -> Float area (Circle _ r) = pi * r ^ 2 area (Rectangle (Point x1 y1) (Point x2 y2)) = (abs $ x2 - x1) * (abs $ y2 - y1) nudge::Shape->Float->Float->Shape nudge (Rectangle(Point x1 y1)(Point x2 y2)) dx dy = Rectangle (Point (x1 +

Look at this output from ghci: Prelude> :t Data.Map.lookup Data.Map.lookup :: Ord k => k -> Data.Map.Map k a -> Maybe a Prelude> :t flip Data.Map.lookup flip Data.Map.lookup :: Ord a => Data.Map.Map a a1 -> a -> Maybe a1 Prelude> let look = flip Data.Map.lookup Loading package array-0.3.0.2 ... linking ... done. Loading package containers-0.4.0.0 ... linking ... done. Prelude> :t look look :: Data.Map.Map () a -> () -> Maybe a Why look 's inferred type differs from type of flip Data.Map.lookup ? To give you some context. Initially I had small program and was trying to figure out why it

Following an example in http://en.wikibooks.org/wiki/Haskell/Beginning Prelude> let abs x = if x < 0 then -x else x Prelude> abs 5 5 Prelude> abs -3 <interactive>:1:6: No instance for (Num (a0 -> a0)) arising from the literal `3' Possible fix: add an instance declaration for (Num (a0 -> a0)) In the second argument of `(-)', namely `3' In the expression: abs - 3 In an equation for `it': it = abs - 3 What's wrong? Haskell thinks you're trying to subtract 3 from abs , and is complaining that abs is not a number. You need to add parenthesis when using the unary negation operator: abs (-3) The

Let's say you want to use your own show function (for example, let show = take 1000 . Prelude.show ). How can you allow ghci to use that for printing instead of the built in show ? Lee You can define your own interactive print function e.g: module BetterPrint betterPrint a = putStrLn (take 1000 $ show a) then start ghci as ghci -interactive-print=BetterPrint.betterPrint 来源： https://stackoverflow.com/questions/35613612/how-can-i-get-ghci-to-use-my-show-function

When I do something simple in ghci, like the following: let x = 7 + 2 I expect ghci to give a response of the type that x holds, like: x :: Integer When I run ghci, I do not get that the above line. How do I get that response? To show types automatically use :set +t : μ> :set +t μ> let x = 7 + 2 x :: Integer μ> Use the ghci :t command, like so: Prelude> let x = 7 + 2 Prelude> :t x x :: Integer Prelude> Taneb To find the type of something in GHCi, you can use the :type command, or (as is much more common), the abbreviated :t . With this, you can do something like: Prelude> let x = 7 + 2 Prelude

Good (your local time of day), everyone. I went through Real World Haskell's chapter on the Foreign Function Interface, and did some follow-up reading here . I'm now experimenting with binding to C functions, and I'd like some clarification on some things. The following is fairly clear: foreign import ccall unsafe "math.h sin" c_sin :: CDouble -> CDouble I can load this and code that uses it in ghci, and everything is fine. It even loads in the embedded ghci in emacs's Haskell mode. I find this great for testing. math is a system library so this is straight-forward. Now an example from Real