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encountered-errors

Encountered Errors​

Put here only if there wasn't an immediate and evident solution after a quick search.

Couldn't match expected type ‘Text’ with actual type ‘[Text]’​

Problem

The basic problem was that the variable known wasn't matching with the list validLocs for some reason. This caused a cascade of problems as I tried a few different variations; indentation would sometimes cause me to go down the wrong path.

{-# LANGUAGE OverloadedStrings #-}
import Data.Text
import Prelude hiding (elem)

validLocs :: [Text]
validLocs = ["cave", "meadow"]

-- ignore leading code
case location of
  known | known `elem` validLocs  -> -- The error arose on this line
-- ignore following code

Solution

The fix was fairly simple: I needed the Prelude version of elem, not the Data.Text version, so changed the import lines:

import Data.Text (Text)         -- explicit import of Text
-- import Prelude hiding (elem) -- removed

Other solutions would work, too, such as using 'qualified' on Data.Text.

Could not load module ‘Data.ByteString.Lazy’​

Problem(s)

From trying the Chris Allen 2014 Tutorial, with regards to the imports.

First, I had to update the .cabal file; this was in the tutorial, but didn't solve the red wiggles in VS Code. A variety of other settings changes didn't do anything. It wasn't until I deleted the .stack-work folder re-ran stack build that the wiggly lines finally resolved.

Had a similar problem with Stackbuilders Scotty project with import Text.Blaze.Html.Renderer.Text, but harder to resolve. I thought adding:

{-# LANGUAGE OverloadedStrings #-}

would do the trick (I missed it originally) since that seems to deal with the Text package, but that didn't do anything.

Update: sometimes the Haskell Language Server gets confused and so the squiggly lines don't automatically clear up. An hie.yaml file will help: https://haskell-language-server.readthedocs.io/en/stable/configuration.html

Solution

What finally fixed it was restarting the extension host.

Hidden packages​

If there is a warning about a hidden package, add that package to the build-depends section of the respective .cabal file.

Could not find module ‘Data.Csv’​

From trying the Chris Allen 2014 Tutorial, with regards to the trying things in stack ghci

Just :quit and relaunch with stack ghci. It looks like it hotloads, but apparently not well enough, or I'm using the wrong commands.

Not in scope: data constructor ‘Optional’​

Probably means you're calling on the type constructor, not the data constructor:

data Optional a = Full a | Empty
mapOptional :: (a -> b) -> Optional a -> Optional b
mapOptional _ Empty = Empty
-- generates error
mapOptional f (Optional a) = Optional(f a)
-- s/b
mapOptional f (Full a) = Full(f a)

Installation Woes​

Tried following https://peerdh.com/blogs/programming-insights/building-a-haskell-based-web-application but right off the bat encountered a problem with cabal while installing yesod. The yesod quick start indicated using stack instead which seemed to do the trick (and took a while).

Had to add the ~/.local/bin file to .zshrc.

Then gave up on that and started from scratch with FPComplete which is nice process, except the page needs to updated because this script is wrong:

#!/usr/bin/env stack
-- stack --resolver lts-13.7 script

main :: IO ()
main = putStrLn "Hello World"

I worked when I replaced lts-13.7 with lts-22.28, which I'm not even sure is the latest, since I got it from some other site as I was trying to figure things out. [ed: latest is 22.35 or stack ls snapshots --lts remote]