I have a fairly complex function with type signature
approxSearchFolder : String -> Nonempty Int -> Int -> Char -> Trie a -> Maybe (ApproxSearchHelpResult Int a) -> Maybe (ApproxSearchHelpResult Int a)
as part of an elm program. When refactoring I changed the type signature to
approxSearchFolder : String -> Nonempty number -> number -> Char -> Trie a -> Maybe (ApproxSearchHelpResult number a) -> Maybe (ApproxSearchHelpResult number a)
i.e. I replaced Int with number. All compiling well so far.
I then changed one of the variables I pass to this function from an Int to a Float. The compiler works out that it is invalid to call this function with some arguments using a number as an Int and others using number as a Float and so gives an error.
The problem, is that the error is not very helpful.
🚨 /home/harry/elm/welsh-whacker/src/Main.elm: Compilation failed
Compiling ...-- TYPE MISMATCH --------------------------------------------------- src/Lib.elm
The 3rd argument to `approxSearchFolder` is not what I expect:
68| (approxSearchFolder
69| word
70| currentRow
71| maxCost
^^^^^^^
This `maxCost` value is a:
number
But ``approxSearchFolder` needs the 3rd argument to be:
Int
Hint: I always figure out the argument types from left to right. If an argument
is acceptable, I assume it is “correct” and move on. So the problem may actually
be in one of the previous arguments!
Hint: The `number` in your type annotation is saying that ints AND floats can
flow through, but your code is saying it specifically wants a `Int` value. Maybe
change your type annotation to be more specific? Maybe change the code to be
more general?
Read <https://elm-lang.org/0.19.1/type-annotations> for more advice!
Detected problems in 1 module.
Now, I had just refactored the type signature of approxSearchFolder so that the 3rd argument could be a number and yet the compiler was telling me that approxSearchFolder's 3rd argument needed to be an Int. But I had just changed that! I spent quite a while trying to make sure that my file watcher was correctly recompiling my code assuming that it was showing an out of date compiler error.
A much more useful error would have been to inform me that the 2nd argument (currentRow) was a Nonempty Int and therefore, for this call of the function numbermust be Int.
Hope this is a helpful thing to catalogue. Thanks!
I have a fairly complex function with type signature
as part of an elm program. When refactoring I changed the type signature to
i.e. I replaced
Intwithnumber. All compiling well so far.I then changed one of the variables I pass to this function from an
Intto aFloat. The compiler works out that it is invalid to call this function with some arguments using anumberas anIntand others usingnumberas aFloatand so gives an error. The problem, is that the error is not very helpful.Now, I had just refactored the type signature of
approxSearchFolderso that the 3rd argument could be a number and yet the compiler was telling me thatapproxSearchFolder's 3rd argument needed to be anInt. But I had just changed that! I spent quite a while trying to make sure that my file watcher was correctly recompiling my code assuming that it was showing an out of date compiler error.A much more useful error would have been to inform me that the 2nd argument (
currentRow) was aNonempty Intand therefore, for this call of the functionnumbermust beInt.Hope this is a helpful thing to catalogue. Thanks!