Compiler hangs and uses more and more memory on type error.

[ajmcmiddlin]

Quick Summary: Compiler hangs and uses more and more memory on type error.

SSCCE

module Main exposing (..)

type alias Lens a b =
    { get : a -> b
    , set : b -> a -> a
    }

type alias Foo =
    { n : Int }

fooN : Lens Foo Int
fooN =
    Lens .n (\foo n -> { foo | n = n })

• Elm: 0.19.1
• Browser: NA
• Operating System: Linux (NixOS 20.09 with kernel 5.4.119)

Additional Details

When I run elm make src/Main.elm the command never returns, and continues to use more and more memory until I kill the process.

The issue is with fooN, as the anonymous function used to define set has its parameters around the wrong way. If we change the definition to fooN = Lens .n (\n foo -> {foo | n = n}) then it compiles without error.

Additionally, if we simplify the Lens type definition to only include set, the compiler produces a type error and doesn't hang.

I originally encountered this using elm-monocle but have inlined the Lens definition to reproduce.

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[bitterjug]

I ran into this very bug this week while using elm-monocle. Looking forward to a fix. But in the meantime I'll switch round the argument order of my getter :blush: