[clean-list] Troubles using a function as parameter to another function

[Vincent Zweije]

Claudio:

||  Start :: *World -> String
||  Start world = processFiles doWork "input.txt" "out.txt" world
||
||
||  processFiles :: (File *File *Files -> (String,*File,*Files)) String String *World -> String
||  processFiles doWork inputName outputName world = result
||  where
||    (result,_) = accFiles doProcess world
||    where
||      doProcess  :: *Files -> (String,*Files)
||      doProcess fileSys
||      # (openInOk,inF,fileSys)  = sfopen inputName FReadText fileSys
||      | not openInOk            = abort ("cannot open input file "+++inputName)
||      # (openOutOk,outF,fileSys)= fopen outputName FWriteText fileSys
||      | not openOutOk           = abort ("cannot open output file
||  "+++outputName)
||      # (result,outF,fileSys)   = doWork inF outF fileSys
||      # (closeOk,fileSys)       = fclose outF fileSys
||      = (result,fileSys)
||  -----------------------------------------------------------------------------
||
||
||  I get this error:
||
||     Uniqueness error [test.icl,5,Start]:"argument 1 of processFiles"
||     attribute at  indicated position could not be coerced
||     (File -> (*File -> ^ *(*Files -> *(String,*File,*Files))))
||
||  I have found a workaround for this by playing around with the definition of
||  the function "doWork", but it is so ugly I don't want to show it here!
||  Another strange fact is that if I defined a function "doWork" that doesn't
||  have the "*Files" parameter, but only the two File, it works!
||
||  I cannot understand why this happen (and the exact meaning of the error
||  message). Am I overlooking something about the uniqueness typing?

My guess is you have hit the murky depths of uniqueness typing and
lifting.

doProcess is a local function.  When it gets lifted, it gets extra
arguments, among which doWork.  The closure using the lifted doProcess
gets passed to accFiles, and I suppose accFiles uses that closure in a
non-unique way.  This would prevent the required uniqueness on doWork,
since it is part of a non-unique structure (the closure).

I'm afraid I can't think of a workaround right now.

Ciao.                                                          Vincent.