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Improve Wire3 error message for opaque types with custom encoders #70

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9dedc48
Improve Wire3 error message for opaque types with custom encoders/dec…
CharlonTank Nov 7, 2025
b01dfdf
Exclude elm/* and lamdera/* packages from custom encoder error
CharlonTank Nov 8, 2025
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37 changes: 36 additions & 1 deletion extra/Lamdera/Wire3/Encoder.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -225,7 +225,42 @@ encoderForType depth ifaces cname tipe =
decoder =
if cname == moduleName
-- Referenced type is defined in the current module
then (a (VarTopLevel moduleName generatedName))
then
-- Check if the generated name exists, if not provide helpful error
case foreignTypeSig moduleName generatedName ifaces of
Just _ -> (a (VarTopLevel moduleName generatedName))
Nothing ->
let
typeNameStr = Data.Name.toChars typeName
customEncoderName = Data.Name.fromChars $ "encode" ++ typeNameStr

-- Only show error for user packages, not elm/* core packages
isUserPackage = case moduleName of
Module.Canonical (Name author _) _ -> author /= "elm" && author /= "lamdera"
in
case (foreignTypeSig moduleName customEncoderName ifaces, isUserPackage) of
(Just _, True) ->
error $ unlines
[ ""
, "-- WIRE3 ENCODER NOT SUPPORTED -----------------------------------------"
, ""
, "I found a custom Wire3 encoder for an opaque type:"
, ""
, " " ++ typeNameStr
, ""
, "This type cannot be used in BackendModel or FrontendModel without"
, "compiler support."
, ""
, "To fix this:"
, ""
, "1. Remove this type from your BackendModel/FrontendModel, OR"
, ""
, "2. Expose the type constructor in the module (less efficient), OR"
, ""
, "3. Add compiler support (contact Lamdera team)"
, ""
]
_ -> (a (VarTopLevel moduleName generatedName))
else (a (VarForeign moduleName generatedName (getForeignSig tipe moduleName generatedName ifaces)))

in
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84 changes: 74 additions & 10 deletions extra/Lamdera/Wire3/Helpers.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -66,19 +66,83 @@ getForeignSig tipe moduleName generatedName ifaces =
-- So add type-sig for failure encoder or decoder as appropriate.
if T.isPrefixOf "w3_encode_" (T.pack $ Data.Name.toChars generatedName)
then
(Forall
(Map.fromList [("a", ())])
(TLambda (TVar "a") tLamdera_Wire_Encoder))
let
genNameStr = Data.Name.toChars generatedName
typeNameStr = drop 10 genNameStr -- Remove "w3_encode_" prefix (10 chars)
customEncoderName = Data.Name.fromChars $ "encode" ++ typeNameStr

-- Only show error for user packages, not elm/* core packages
isUserPackage = case moduleName of
Module.Canonical (Name author _) _ -> author /= "elm" && author /= "lamdera"
in
case (foreignTypeSig moduleName customEncoderName ifaces, isUserPackage) of
(Just _, True) ->
error $ unlines
[ ""
, "-- WIRE3 ENCODER NOT SUPPORTED -----------------------------------------"
, ""
, "I found a custom Wire3 encoder for an opaque type:"
, ""
, " " ++ typeNameStr
, ""
, "This type cannot be used in BackendModel or FrontendModel without"
, "compiler support."
, ""
, "To fix this:"
, ""
, "1. Remove this type from your BackendModel/FrontendModel, OR"
, ""
, "2. Expose the type constructor in the module (less efficient), OR"
, ""
, "3. Add compiler support (contact Lamdera team)"
, ""
]
_ ->
(Forall
(Map.fromList [("a", ())])
(TLambda (TVar "a") tLamdera_Wire_Encoder))

else if T.isPrefixOf "w3_decode_" (T.pack $ Data.Name.toChars generatedName)
then
(Forall
(Map.fromList [("a", ())])
(TAlias
mLamdera_Wire
"Decoder"
[("a", TVar "a")]
(Filled (TType (Module.Canonical (Name "elm" "bytes") "Bytes.Decode") "Decoder" [TVar "a"]))))
let
genNameStr = Data.Name.toChars generatedName
typeNameStr = drop 10 genNameStr -- Remove "w3_decode_" prefix (10 chars)
customDecoderName = Data.Name.fromChars $ "decode" ++ typeNameStr

-- Only show error for user packages, not elm/* core packages
isUserPackage = case moduleName of
Module.Canonical (Name author _) _ -> author /= "elm" && author /= "lamdera"
in
case (foreignTypeSig moduleName customDecoderName ifaces, isUserPackage) of
(Just _, True) ->
error $ unlines
[ ""
, "-- WIRE3 DECODER NOT SUPPORTED -----------------------------------------"
, ""
, "I found a custom Wire3 decoder for an opaque type:"
, ""
, " " ++ typeNameStr
, ""
, "This type cannot be used in BackendModel or FrontendModel without"
, "compiler support."
, ""
, "To fix this:"
, ""
, "1. Remove this type from your BackendModel/FrontendModel, OR"
, ""
, "2. Expose the type constructor in the module (less efficient), OR"
, ""
, "3. Add compiler support (contact Lamdera team)"
, ""
]
_ ->
(Forall
(Map.fromList [("a", ())])
(TAlias
mLamdera_Wire
"Decoder"
[("a", TVar "a")]
(Filled (TType (Module.Canonical (Name "elm" "bytes") "Bytes.Decode") "Decoder" [TVar "a"]))))
else
error $ "impossible getForeignSig on non-wire function: " ++ Data.Name.toChars generatedName

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