Type error being deferred to other values
Summary
The following code doesn't type check because of the last line <> radius
, which has type Double
while the rest of the values have type Diagram B
.
{# LANGUAGE NoMonomorphismRestriction #}
{# LANGUAGE TypeFamilies #}
module Test where
import Diagrams.Prelude
import Diagrams.Backend.SVG.CmdLine
someFunc :: Double > Diagram B
someFunc radius = result
where
commonSubExpr = circle radius
unrelatedValue
= commonSubExpr
<> (circle 5 :: Diagram B)
result
= commonSubExpr
<> (circle 1 :: Diagram B)
<> radius
However, GHC prints out type errors on almost every value but the radius:
...\Test.hs:9:19: error:
* Couldn't match type `Double' with `QDiagram B (V B) (N B) Any'
Expected type: Diagram B
Actual type: Double
* In the expression: result
In an equation for `someFunc':
someFunc radius
= result
where
commonSubExpr = circle radius
unrelatedValue = commonSubExpr <> (circle 5 :: Diagram B)
result = commonSubExpr <> (circle 1 :: Diagram B) <> radius

9  someFunc radius = result
 ^^^^^^
...\Test.hs:15:16: error:
* Couldn't match type `QDiagram B (V B) (N B) Any' with `Double'
Expected type: Double
Actual type: Diagram B
* In the second argument of `(<>)', namely
`(circle 5 :: Diagram B)'
In the expression: commonSubExpr <> (circle 5 :: Diagram B)
In an equation for `unrelatedValue':
unrelatedValue = commonSubExpr <> (circle 5 :: Diagram B)

15  <> (circle 5 :: Diagram B)
 ^^^^^^^^^^^^^^^^^^^^^
...\Test.hs:19:17: error:
* Couldn't match type `QDiagram B (V B) (N B) Any' with `Double'
Expected type: Double
Actual type: Diagram B
* In the first argument of `(<>)', namely `(circle 1 :: Diagram B)'
In the second argument of `(<>)', namely
`(circle 1 :: Diagram B) <> radius'
In the expression:
commonSubExpr <> (circle 1 :: Diagram B) <> radius

19  <> (circle 1 :: Diagram B)
 ^^^^^^^^^^^^^^^^^^^^^
where QDiagram B (V B) (N B) Any
is what Diagram B
resolves to.
There is a lot of odd with this. For one, GHC seems convinced that the type of result
is Double
even though Double
doesn't even have a Semigroup
instance. Secondly, the type error seems to spread through commonSubExpr
into unrelatedValue
. Additionally circle x
returns a value from a typeclass called TrailLike
, but the typeclass doesn't have an instance for Double
, so it's a bit odd that GHC doesn't have a problem with the type of commonSubExpr
.
Also, if you replace the definition of commonSubExpr
with commonSubExpr = circle (1.2 :: Double)
, GHC will stop reporting the second type error even though the types of all expresions should be the same.
Steps to reproduce
Compile the above code along with the diagramslib, diagrams and diagramssvg libraries.
Expected behavior
I expected GHC to give an error about the right and left arguments of the last <>
being different, thus pointing directly to the source of the type error.
Environment
 GHC version used: 8.6.5
 Diagrams version used: 1.4 (for all 3 libraries)
Optional:
 Operating System: Windows 10
 System Architecture: x8664