---
abstract: Rant about Haskell's parser "Happy".
lastupdated: 2014
---
# Unhappy About Happy: A Reflection or Something Like That
*2014.*
For the purposes of my quiet rage, this document will be written in all
caps. I also will not use proper language. I won't even give suggestions
on how to improve Happy; I'll just complain.
STARTING NOW
WHAT HAPPY ???
--------------
HAPPY IS PARSER GENERATOR. IT GENERATE HASKELL CODE. VISIT
[](https://www.haskell.org/happy/) FOR
UGLY INFO.
HOW HAPPY GOOD
--------------
- EFFICIENT I GUESS
- WORKS OK
- PRETTY LOGO
WHY I NOT LIKE HAPPY
--------------------
### JUZT TYPICAL
ONE ERROR IN PARSER.Y GIVE HUGE TYPE ERROR
**WHAT IS THIS!**
dist/build/l0c/l0c-tmp/Language/L0/Parser/Parser.hs:2452:42:
Couldn't match type `Value' with `BasicValue -> Value'
Expected type: HappyAbsSyn
(Name, SrcLoc)
[(Name,
DeclTypeBase Name,
[ParamBase Name],
ExpBase NoInfo Name,
SrcLoc)]
(Name,
DeclTypeBase Name,
[ParamBase Name],
ExpBase NoInfo Name,
SrcLoc)
Uniqueness
([Maybe (DimSize Name)], ElemTypeBase NoInfo Name)
(ElemTypeBase NoInfo Name)
[TypeBase NoInfo Name]
[TypeBase NoInfo Name]
[ParamBase Name]
(Maybe (CertificatesBase NoInfo Name), [ExpBase NoInfo Name])
[ExpBase NoInfo Name]
[ExpBase NoInfo Name]
/* ... 200-300 MORE LINES ... */
[TupIdentBase NoInfo Name]
(TupIdentBase NoInfo Name)
(LambdaBase NoInfo Name)
(TupleLambdaBase NoInfo Name)
(BasicValue -> Value)
Int
(BasicValue -> Value)
(BasicValue -> Value)
(BasicValue -> Value)
(BasicValue -> Value)
(BasicValue -> Value)
Value
(BasicValue -> Value)
(BasicValue -> Value)
[Value]
[Value]
In the second argument of `happySpecReduce_3', namely
`happyReduction_191'
In the expression: happySpecReduce_3 38# happyReduction_191
In an equation for `happyReduce_191':
happyReduce_191 = happySpecReduce_3 38# happyReduction_191
**NOT NICE NOT NICE AT ALL**
### MONADA CAN I HAZ
MAYHAPS U NEED TO PARSE DIFFERENT THAN NORMAL KAY. HOW U THINK. YES:
MENTALINCRMENTAL PARSING IT IS
YOU GOT NEED TO PARSE IO GETLINE SOURCES YEAH
PARSE EXPRESSION "2 + 1 \*4" OVER MULTI LINES LIKE THIS HERE
WELCOM TO DA FUNKY MULTILINE EXPRESSION PARSER; ENTER UR EXPRESSION
> 2 +
1 *4
TY GOT IT!! PARSED EXPRESSION IS "2 + 1 * 4" AND U NICE; RESULT IS 6 BTW
>
PARSER SHOULD KNOW "YEAH NOT DUHNE PARSIN YET" WHEN "2 +" READ, SHOULD
TEHN READ NEW LINE AND PARSE MORE!
WE CAN PARSE FROM BEGINNING EVERY TIME SURE BUT INEFECICIENT!!!
BUT HOW WE SHOULD / COULD DO THAT SMARTLY I IMPLORE TO LIKE ASK U
YES WE USE CONTINUATION WHICH SMART. LOOK ATTOPARSEC USE CONTINUATION IN
RETURN!
[](https://hackage.haskell.org/package/attoparsec-0.11.1.0/docs/Data-Attoparsec-Text.html#t:IResult)
YEAH BUT NOT IN HAPPY............
SO WE MAKE IT MANUAL. WE SPECIFY OWN LEXER AND IMPROVE MONAD. WHEN END
OF LINE IS REACH, WE TELL PARSER (FROM INSIDE DA PRASER) DAT DERE IS NO
MORE EVER; IF THAT FAILS WE CATCH ERROR (EITHER-LIKE MONAD) AND READ NEW
LINE FOR REALS
CLEVER SOLUTION OF US! BUT IN PARSE EXPRESION EXAMPLE ABOV WE USE IO TO
GETLINE
**WE NOT WANT IO IN ALL PARSING.... EVER EVER!!!!!!!!**
HOWTO ABSTRACT IO AWAY? WE WANT TYPE OF EXPORTED FUNCTIONS FROM PARSER
TO NOT USE **IO** *AT ALL*
SO WE USE WHATEVER MONAD TO GETLINE RIGHT?
``` {.haskell}
class GetLiner m where -- KAY SO m IS MONAD RIGHT
getSomeLine :: m String
instance GetLiner IO where
getSomeLine = getLine
```
WAIT I DONT GET IT
``` {.haskell}
CLASS GETLINER M WHERE -- KAY SO M IS MONAD RIGHT
GETSOMELINE :: M STRING
INSTANCE GETLINER IO WHERE
GETSOMELINE = GETLINE
```
MUCH BETTR. COZ TEHN WE CAN USE
BUT YIAH WE COULD USE TYPE CONSTRAINTS!
``` {.haskell}
LEXER :: GETLINER M => M SOMETHNIG
LEXER = MEHMEHMEH
WOO <- GETSOMELINE
MEH MEH
```
SURE COULD WORK WE NEED JUST TELL HAPPY BOUT IT.
%MONAD { GETLINER M => SOMETHINNG M }
FAILS MISERBLY.................... :-( :( :(
WE ARE LEFT 4 PONDER HOW TO SOLVE! HOWTO ABSTRAKT OUT IO! HOW!!
SOOOO..... WE NEED KINDA CONTINUATIN COMPUTATON MONADY THINGY. I GOT
FRIEND WHO COME WIT CODE, SAY IT GOOD CODE. I CHECK, HE RIGHT! IS HERE
``` {.haskell}
DATA READLINEMONAD A = VALUE A
| GETLINE (STRING -> READLINEMONAD A)
INSTANCE MONAD READLINEMONAD WHERE
RETURN = VALUE
VALUE X >>= F = F X
GETLINE G >>= F = GETLINE $ \S -> G S >>= F
```
HA CLEVER!!! NOW WE NEED NO TYPE CONSTRAINTS BECAUSE NOW COMPUTATION IS
MONAD ITSELF! HOW YOU ASK.
READLINEMONAD HAS NO IO OR ANY ELSE BUILTIN; IT GETS LINE BY CREATE
FUNCTION WHICH TAKES LINE TO CONTINUU EXECUTION. AGAIN AND AGAIN! LINE
CAN COME FROM ANYTHIN.
EXAMPLE FOR USING RESULT!
``` {.haskell}
GETLINESFROMIO :: READLINEMONAD A -> IO A
GETLINESFROMIO (VALUE X) = RETURN X
GETLINESFROMIO (GETLINE F) = DO
S <- GETLINE
GETLINESFROMIO $ F S
```
U SEE WAT GOIN ON DERE! IN OUR LEXER WE NAMELI USED
``` {.haskell}
GETLINE VALUE
```
TO GET LINE (BECAUSE FITS WITH CLEVER MONAD!)
**NICE-O!**
CODE IN QUSTION
---------------
LOOKIE HER FOR FULL DETAILS (ALTHOU WITH NO FULL CAPS):
[](https://raw.github.com/HIPERFIT/L0Language/master/src/Language/L0/Parser/Parser.y)
CONCLUSION
----------
THIS IS ENTRELY OBJECTIVE.
THIS IS ENTIREY OBJECTIVE.
THIS IS ENTIRLY OBJECTIVE.