CODEWITHSUNDEEP
debugginghaskelltypes
altern
altern

Reputation: 5949

Haskell: is it possible to output type as a part of a program?

To help with debugging and writing programs in Haskell, I am thinking about ability of Haskell programs to output types of variables as part of the program. For example, I have following code:

listHEADFiles :: ReaderT LgRepo IO ()
listHEADFiles = do
  ref <- resolveReference $ T.pack "HEAD"
  case ref of 
    Nothing -> fail "Could not resolve reference named 'HEAD'"
    Just reference -> do
      obj <- lookupObject reference
      case obj of 
        CommitObj commit -> do
          objects <- listAllObjects Nothing (commitOid commit)
          for_ objects (\case 
            TreeObjOid toOid -> do
              tree <- lookupTree toOid
              treeEntries <- sourceTreeEntries tree
              entries <- lift $ treeEntries
              outputTypeOf entries
            )
        _ -> fail "'HEAD' is not a commit object"

I want to output type of variable entries because I fail to understand what exactly happens after I lift the value. I can go to documentation, but it always perplexes me to calculate it by hand. I would like to know for sure what type it is when my program is executed. In other words, I want functionality of :t in ghci as a part of my program. Is it possible?

Upvotes: 3

Views: 55

Answers (1)

amalloy
amalloy

Reputation: 92117

You don't really want your program to output a type: you want the compiler to output a type when it compiles your program. The feature you're looking for is Partial type signatures. The idea is you put an incomplete signature on an expression, and you get out a compiler "error" telling you how to fill in the blanks. If you have no idea at all of the type, an acceptable incomplete signature would be just _:

(entries :: _) <- lift $ treeEntries

Upvotes: 4

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