User:Zzo38/Proposal for macros
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(New page: This document list proposal for use of macros in Haskell. =Defining macros= =Macros for catching pattern failure= Example: a :: Int -> Int -> Int; a 0 x = x; a 1 x = x + x; b :: Int...) |
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Example: | Example: | ||
| + | {-# LANGUAGE Macro #-} | ||
| + | import Control.Exception.Macro; | ||
a :: Int -> Int -> Int; | a :: Int -> Int -> Int; | ||
a 0 x = x; | a 0 x = x; | ||
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d 0 = return id; | d 0 = return id; | ||
f = fail "undefined"; | f = fail "undefined"; | ||
| + | |||
| + | Note that <tt>_Catch</tt> and so on are macros, not functions. A macro is not a first-class value in the program, and cannot be used at run-time, but can have operations that a function doesn't have. | ||
| + | |||
| + | When calling the new function created by the macro, errors are converted to the return value instead (a function could not read undefined values, but a macro could change it into a different function so that it is not undefined). | ||
| + | |||
| + | You could specify strictifying with do-notation to ensure everything in there is defined before returning a value, so that an undefined value will be caught ahead of time. | ||
| + | |||
| + | User errors and pattern match errors can be caught even in a pure code, but out of memory error can only be caught in I/O actions. | ||
Revision as of 20:01, 12 January 2012
This document list proposal for use of macros in Haskell.
1 Defining macros
2 Macros for catching pattern failure
Example:
{-# LANGUAGE Macro #-}
import Control.Exception.Macro;
a :: Int -> Int -> Int;
a 0 x = x;
a 1 x = x + x;
b :: Int -> Int -> Either String Int;
b = _Catch 2 a;
c :: Int -> Int -> IO Int;
c = _CatchIO 2 a;
d :: Int -> IO (Int -> Int);
d = _CatchIO 1 a;
e :: [String];
e = ["hello", "world", undefined];
f :: IO [String];
f = _CatchStrictIO 0 e do {
case _This of {
[] <- _Stop;
h : t <- do {
h;
_Again t;
};
};
};
Meanings:
b 0 = Right; b 2 = const $ Left "pattern match error"; c 0 = return; c 2 = const $ fail "pattern match error"; d 0 = return id; f = fail "undefined";
Note that _Catch and so on are macros, not functions. A macro is not a first-class value in the program, and cannot be used at run-time, but can have operations that a function doesn't have.
When calling the new function created by the macro, errors are converted to the return value instead (a function could not read undefined values, but a macro could change it into a different function so that it is not undefined).
You could specify strictifying with do-notation to ensure everything in there is defined before returning a value, so that an undefined value will be caught ahead of time.
User errors and pattern match errors can be caught even in a pure code, but out of memory error can only be caught in I/O actions.
