TransformListComp weird type error

Am I missing something, or is this broken?

Prelude> :set -XTransformListComp 
Prelude> import Data.List
Prelude Data.List> [ x | x <- ['a'..'z'], then sort]

<interactive>:5:29:
    No instance for (Ord a) arising from a use of `sort'
    Possible fix:
      add (Ord a) to the context of
        a type expected by the context: [a] -> [a]
    In the expression: sort
    In a stmt of a list comprehension: then sort
    In the expression: [x | x <- ['a' .. 'z'], then sort]

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
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Architecture

changed milestone to %7.8.1

changed weight to 5

added Tbug Trac import typechecker labels

Broken or not, it's documented:

• *then f** — This statement requires that f have the type forall a. [a] -> [a]

Ok, I guess that makes sense if we're typechecking the concrete syntax before desugaring. But I was using MonadComprehensions, and hence was looking at that section of the documentation which says

[ x+y | x <- [1..10], y <- [1..x], then take 2 ]

This translates to:

do (x,y) <- take 2 (do x <- [1..10]
                       y <- [1..x]
                       return (x,y))
   return (x+y)

So I found the behaviour quite surprising.

closed

It's not a bug! There's a good reason for it, which you'll find discussed in the paper.

That said, I can see it's inconvenient here, and others may have ideas about what to do about it.

Simon

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedInvalid

added Pnormal label