When attempting to load Haskell file test.hs into terminal. I get this response:
<interactive>:5:1: error: Variable not in scope: test :: b0 -> c
<interactive>:5:6: error: Variable not in scope: hs :: a -> b0
This is the code from test.hs:
double x = x + x
quadruple x = double (double x)
How do I get the file to load in terminal?
openai says look for documentation or may need to import module?
That's what happens if you type test.hs at the GHCi prompt. You meant to type :l test.hs instead.
Related
I am starting in Haskell and encounter this weird error message when trying to compile a .ghci file in Haskell. I have this very simple code for example:
main = do
putStrLn "Greetings! What is your name?"
inpStr <- getLine
putStrLn $ "Welcome to Haskell, " ++ inpStr ++ "!"
I saved the code in a file called basicio.hs and tried to run
ghc basicio.hs
Instead of the String I get the following message
<interactive>:2:1: error:
Variable not in scope: runghc :: t0 -> b0 -> c
<interactive>:2:8: error: Variable not in scope: basicio
I am not sure what's wrong, the command ":load" works fine and find my file.
This error messages looks like it was generated by typing "runghc basicio" at the GHCi prompt:
GHCi, version 8.6.5: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from ...
> runghc basicio
<interactive>:3:1: error: Variable not in scope: runghc :: t0 -> t
<interactive>:3:8: error: Variable not in scope: basicio
>
However, the runghc command, and the compiler command ghc are both meant to be run directly from the command line.
The problem I am facing is running from an external file
But I am getting the error "Variable not in scope"
I am running on windows and its the most basic program. When I am trying to do this fully by command prompt it is working fine. But when I am trying to do this from an external file it is giving this error
file name: haskell_trial.hs
doubleMe x = x + x
While running this is the error:
Prelude> :l haskell_trial
[1 of 1] Compiling Main ( haskell_trial.hs, interpreted )
Ok, one module loaded.
*Main> doubleMe 5
<interactive>:6:1: error:
Variable not in scope: doubleMe :: Integer -> t
This is what i get when i simply do it from command prompt:
Prelude> doubleme x = x+x
Prelude> doubleme 9
18
It was some setup issue. I installed again and it started working fine
I'm trying to execute a main method within one of my classes in haskell. When I run the command runhaskell mod12PA.hs I get an error and I can't figure out why.
Here's the code:
-- The main program: read points from stdin, write an SVG file to stdout.
main :: IO ()
main = do
putStrLn "Enter points: "
points <- getLine
putStrLn ("Points: " ++ points)
The error:
<interactive>:41:1: error:
Variable not in scope: runhaskell :: t0 -> b0 -> c
<interactive>:41:12: error: Variable not in scope: mod12PA
<interactive>:41:20: error: Variable not in scope: hs :: a -> b0
Was able to figure out that I needed to be compiling the file outside of ghci and then running the executable. I was trying to execute the file from within ghci which was not working.
Ran ghc --make mod12PA.hs
Then mod12PA.exe
While you do need to run the command outside of GHCi, runhaskell mod12PA.hs should work. (runhaskell serves a different purpose than ghc --make: it runs the code without compiling it.)
In Haskell using GHCi is there a way to load a file such as the one below, that lets you test the methods that have bindings.
Use case: trying to test part of my module, while having skeleton code for the rest of it. (as to not have an XY problem)
module X (methodA, methodB, methodC) where
methodA :: String->String
methodA name = "Hello " ++ name
methodB :: Int -> String
methodC :: String -> String
This obviously outputs the correct error : The type signature for ‘methodB’ lacks an accompanying binding.
For example I would like something similar to below to work.
GHCi, version 8.0.1: http://www.haskell.org/ghc/ :? for help
Prelude> :l example.hs
[1 of 1] Compiling X ( example.hs, interpreted )
example.hs:6:1: error:
The type signature for ‘methodB’ lacks an accompanying binding
example.hs:8:1: error:
The type signature for ‘methodC’ lacks an accompanying binding
Failed, modules loaded: none.
Prelude> methodA "jamesmstone"
<interactive>:2:1: error:
Variable not in scope: methodA :: [Char] -> t
You can use -fdefer-type-errors. This will give you warnings about type errors but won't prevent you from running the other well-typed parts of your program.
Example:
If Program.hs contains:
foo :: Int
foo = 'a'
main = putStrLn "Hello, world"
Then with -fdefer-type-errors you can still load and run main:
$ ghci -fdefer-type-errors Program.hs
GHCi, version 7.10.2: http://www.haskell.org/ghc/ :? for help
[1 of 1] Compiling Main ( Program.hs, interpreted )
Program.hs:5:7: Warning:
Couldn't match expected type ‘Int’ with actual type ‘Char’
In the expression: 'a'
In an equation for ‘foo’: foo = 'a'
Ok, modules loaded: Main.
*Main> main
Hello, world
*Main>
Just give them a binding that doesn't work:
methodB = undefined
methodC = undefined
Simple question. Is it possible to check the type of a variable that is only alive within a function?
For example:
main = do
x <- something
How can I check the type of x?
I can't do :type x in ghci because x is not global.
Another way (quite similar to RobAgar's and hacky as well) is to explicitly specify some wrong type for the local variable in question, e.g.:
main = do
x <- getArgs
x :: () -- let's cast it to unit! ;)
print $ head x
Then ghci will give us an error:
Couldn't match expected type `()' with actual type `[String]'
which shows that the actual type of "x" is [String].
There is a hacky way:
main = do
x <- something
foo x
where foo is any old function which doesn't take an argument of the type you think x might be. You'll get a nice error from GHC which includes the type expected by foo and the type actually passed in.
You can use the GHCi Debugger:
> ghci a.hs
GHCi, version 7.0.4: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
[1 of 1] Compiling Main ( a.hs, interpreted )
Ok, modules loaded: Main.
*Main> :break 4
Breakpoint 0 activated at a.hs:4:8-14
*Main> :main
Stopped at a.hs:4:8-14
_result :: IO String = _
[a.hs:4:8-14] *Main> :list
3 main = do
4 x <- getLine
^^^^^^^
5 return x
[a.hs:4:8-14] *Main> :step
asd
Stopped at a.hs:5:3-10
_result :: IO String = _
x :: String = _
[a.hs:5:3-10] *Main> :t x
x :: String
There is no easy way to do this. If your something function is in the global scope, you can check the type of that function. If it really is in your main function, its type is going to be IO SomeType, where SomeType is probably what you're looking for.
Another option is Scion which is basically an external wrapper over the GHC api which acts as a server providing IDE-like capabilities for editors like Emacs and Vim.
In the readme, it mentions the "experimental" command C-c C-t which shows the type of the identifier at point, including local identifiers. However, this will only work if your file type-checks.
This lets you find out the type of a local declaration without compiling your file or loading into GHCi, which means it won't disrupt your flow of thought as much.
This is sort of the obvious non-answer. Given a local binding of the form
x <- something
In order to know the type of x, you merely need to know the type of something. Given something has the type m a (for some Monad m), x must therefore have the type a.