On 12/6/10 7:26 PM, Daryoush Mehrtash wrote:
I am having hard time understanding the following code. The code is from
Applicative Parser library:
http://hackage.haskell.org/packages/archive/uu-parsinglib/2.5.5.2/doc/html/src/Text-ParserCombinators-UU-BasicInstances.html
instance (Show a, loc `IsLocationUpdatedBy` a) => Provides (Str a
loc) (a -> Bool, String, a) a where
splitState (p, msg, a) k (Str tts msgs pos del_ok)
= let ins exp = ...1
del exp = ...2
in case tts of
(t:ts) -> if p t
then ...3
else Fail [msg] (*ins:* if *del_ok* then
*[del*] else [])
[] -> Fail [msg] [*ins*]
Specifically I am having hard time understanding how the "ins exp" and "del
exp" in the "let" is related to "ins:" "del_ok", [del], [ins].
I don't understand how given the "let" expression the following expressions
are expanded
The let expressions are binding functions so ins names (\exp -> ...1)
and del names (\exp -> ...2). Thus, when del_ok is true we get,
Fail [msg] (ins : if del_ok then [del] else [])
==
Fail [msg] (ins : if del_ok then del : [] else [])
==
Fail [msg] (ins : del : [])
==
Fail [msg] ((\exp -> ...1) : (\exp -> ...2) : [])
and when del_ok is false we get
Fail [msg] ((\exp -> ...1) : [])
--
Live well,
~wren
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