[clean-list] Two questions (Types and graphics)

[John van Groningen]

Jerzy Karczmarczuk wrote:
>...
>I defined a linear interpolation operator:
>
>  linterp i0 i1 t 
>   | t<zero = i0
>   | t>one = i1
>   | otherwise = (one-t)*>i0 + t*>i1
>
>and I put it into the main module, in order to request from the compiler
>its type. You, guru folks do the same, I see it from the fact that
>plenty of your declaration and implementation modules have
>trailing, useless semicolons in type declarations...
>
>I got
>
>  linterp :: (a b) (a b) b -> a b | * , - , < , one , zero b & *> a;

The compiler has derived an incorrect type. The context restriction +(a b)
is missing. It should be:

linterp :: (a b) (a b) b -> a b | * , - , < , one , zero b & *> a & + (a b);

>and using the marvelous lazy functional technique called cut-and-paste,
>I transferred linterp into my shared library.
>
>Then I recompiled the main module and its subsidiaries, and I got
>
>Type error [Veclib.icl,87,linterp]: 
>  "+" requested instance of the form (m a1 .. ak) is impossible

This is correct, because + (a b) is missing.

Unfortunately such contexts are not supported by Clean 1.3.3.

>...
>
>A few other questions.
>
>How to *READ A PIXEL* from a Picture?
>Even simpler: how to read a pixel value from a bitmap?

This is not implemented in ObjectIO.

>I remember the answers of Peter Achten some months ago. Picture is
>an output-only structure etc. Not sure, what a pixel is, etc. do
>everything yourself. We would like to, not our priority, etc. 
>Frankly, not very promising answers. 
>
>I'll tell you what I tried concerning bitmaps. First I moved to a 
>Windows (NT) machine. Then, I opened the file using openbitmap, 
>and recovered the OsBitmap structure.
>
>I got several pieces of data, for example the size, say, 284*281 
>of my  BMP picture. And the {#Char} BitmapContent as well. But it 
>is not directly usable.
> 
>I know, the data in a BMP and the pixel array are different things. 
>I looked into pictCCall_12.icl, but from your inline codes one 
>cannot learn anything. HDC is just an integer, the access routines 
>are opaque. What can I do with the bitmapHandle? Where is the
>palette? (Suppose I want to do some colour cycling.)

The inline codes are calls to functions written in C. The code of the
functions using bitmaps can be found in the file 'cpicture_12.c'
in the directory 'OS Windows/Windows_C_12' of Object IO 1.2.1.

>I know (more or less) the BMP file format. Shall I process the 
>BMP contents by hand? It would be utterly silly, Windows did it 
>for me. I checked as far as I could the Windows programming sections 
>in IO lib, nothing found.
>
>I wonder whether you have ever documented this low-level side of 
>Clean IO programming in order to ENABLE OTHERS TO DO SOME WORK? 

As far as I know this has not been documented.

>...
>Why don't you use some DLLs instead of your inline codes? It
>might then be easier to make it more portable.
>...

The inline codes are calls to C functions, putting this C code in a DLL instead
of an object file doesn't make it easier to change this code.

Regards,
John van Groningen