stricter-labelled-field-syntax.md

+# Proposal: [StricterLabelledFieldSyntax](stricter-labelled-field-syntax)
+
+<table><tr><th> Ticket       </th>
+<th>[\#132](https://gitlab.haskell.org//haskell/prime/issues/132)</th></tr>
+<tr><th> Dependencies </th>
+<th> none 
+</th></tr>
+<tr><th> Related      </th>
+<th> none 
+</th></tr></table>
+
+## Compiler support
+
+<table><tr><th> GHC   </th>
+<th> partial (test patch available) 
+</th></tr>
+<tr><th> nhc98 </th>
+<th> none 
+</th></tr>
+<tr><th> Hugs  </th>
+<th> none 
+</th></tr>
+<tr><th> UHC   </th>
+<th> none 
+</th></tr>
+<tr><th> JHC   </th>
+<th> none 
+</th></tr>
+<tr><th> LHC   </th>
+<th> none 
+</th></tr></table>
+
+## Summary
+
+
+Make the labelled field syntax stricter, so that unclear code is illegal.
+
+## Description
+
+
+Many people believe that the precedence of labelled fields creation, updates and pattern matching can lead to confusing code. For example, in [ http://hackage.haskell.org/trac/ghc/ticket/2530](http://hackage.haskell.org/trac/ghc/ticket/2530) it was reported that this program:
+
+```wiki
+module Main where
+
+data A = A {x :: Int} deriving (Show)
+
+main :: IO ()
+main = print $ Just A {x = 5}
+```
+
+
+(correctly, according to Haskell 98) prints
+
+```wiki
+Just A {x = 5}
+```
+
+
+in hugs, but the more easily comprehendible
+
+```wiki
+Just (A {x = 5})
+```
+
+
+in ghci.
+
+
+According to Haskell 98,
+
+```wiki
+A {x = 5}
+```
+
+
+is an atomic expression - but it doesn't look atomic! This violates the principle of least surprise.
+
+
+Before coming across labelled fields, but having had "function application binds tightest" drummed into you to understand how to read expressions containing a mixture of applications and infix operators, of the following 2 functions:
+
+```wiki
+data A = A {x :: Bool} deriving (Show)
+
+f = print $ Just A {x = True}
+g = print $ A True {x = False}
+```
+
+
+I would expect `g` to be the correct one, while it is `f` that is correct according to Haskell 98.
+
+
+However, once you remove the space before the curly brace:
+
+```wiki
+h = print $ Just A{x = True}
+i = print $ A True{x = False}
+```
+
+
+it is `h` that looks more correct. Note that we have a similar problem with infix operators, and expressions like
+
+```wiki
+1+f x+2
+```
+
+
+I propose that all of `f`, `g`, `h` and `i` be made illegal, with parentheses being required to disambiguate these cases.
+
+
+Here are some real-life examples of what I consider confusing code, from `haskeline`:
+
+```wiki
+metaKey :: Key -> Key
+metaKey (Key m bc) = Key m {hasMeta = True} bc
+```
+
+```wiki
+searchText :: SearchEntry -> [Grapheme]
+searchText SearchEntry {entryState = IMode xs ys} = reverse xs ++ ys
+```
+
+
+and from `Cabal`:
+
+```wiki
+configure ... = do
+  ...
+  (ghcPkgProg, ghcPkgVersion, conf'') <-
+    requireProgramVersion verbosity ghcPkgProgram {
+      programFindLocation = guessGhcPkgFromGhcPath ghcProg
+    }
+    anyVersion (userMaybeSpecifyPath "ghc-pkg" hcPkgPath conf')
+  ...
+```
+
+```wiki
+buildExe :: Verbosity -> PackageDescription -> LocalBuildInfo
+                      -> Executable         -> ComponentLocalBuildInfo -> IO ()
+buildExe verbosity _pkg_descr lbi
+  exe@Executable { exeName = exeName', modulePath = modPath } clbi = do
+  ...
+```
+
+```wiki
+      case PackageIndex.lookupPackageName index (PackageName "rts") of
+        [rts] -> PackageIndex.insert rts { Installed.ldOptions = [] } index
+```
+
+```wiki
+register pkg@PackageDescription { library       = Just lib  }
+         lbi@LocalBuildInfo     { libraryConfig = Just clbi } regFlags
+    = ...
+```
+
+## References
+
+
+A patch to add support for the syntax to GHC, as well as patches needed to fix GHC and the libraries to follow the new syntax, are in the ticket ([\#132](https://gitlab.haskell.org//haskell/prime/issues/132)).
+
+## Report Delta
+
+
+In [ Section 3](http://haskell.org/onlinereport/exps.html#sect3) replace:
+
+```
+
+exp10 -> ...
+       | fexp
+```
+
+
+with:
+
+```
+
+exp10 -> ...
+       | recexp
+recexp -> qcon { fbind1 , ... , fbindn }        (labeled construction, n >= 0)
+        | aexp<qcon> { fbind1 , ... , fbindn }  (labeled update, n >= 1) 
+        | fexp
+```
+
+
+and remove:
+
+```
+
+aexp -> ...
+      | qcon { fbind1 , ... , fbindn }        (labeled construction, n>=0)
+      | aexp<qcon> { fbind1 , ... , fbindn }  (labeled update, n >= 1)
+```
+
+
+In [ Section 3.15.2](http://haskell.org/onlinereport/exps.html#sect3.15.2) replace:
+
+```
+
+aexp -> qcon { fbind1 , ... , fbindn }        (labeled construction, n>=0)
+```
+
+
+with:
+
+```
+
+recexp -> qcon { fbind1 , ... , fbindn }        (labeled construction, n >= 0)
+```
+
+
+In [ Section 3.15.3](http://haskell.org/onlinereport/exps.html#sect3.15.3) replace:
+
+```
+
+aexp -> aexp<qcon> { fbind1 , ... , fbindn }  (labeled update, n >= 1)
+```
+
+
+with:
+
+```
+
+recexp -> aexp<qcon> { fbind1 , ... , fbindn }  (labeled update, n >= 1)
+```
+
+
+In [ Section 3.17.1](http://haskell.org/onlinereport/exps.html#sect3.17.1) replace:
+
+```
+
+pat10 -> ...
+```
+
+
+with:
+
+```
+
+pat10 -> ...
+       | qcon { fpat1 , ... , fpatk }    (labeled pattern, k >= 0)
+```
+
+
+and remove:
+
+```
+
+apat -> ...
+      | qcon { fpat1 , ... , fpatk }    (labeled pattern, k>=0)
+```
+
+
+In [ Section 9.5](http://haskell.org/onlinereport/syntax-iso.html#sect9.5) replace:
+
+```
+
+exp10 -> ...
+       | fexp
+```
+
+
+with:
+
+```
+
+exp10 -> ...
+       | recexp
+recexp -> qcon { fbind1 , ... , fbindn }        (labeled construction, n >= 0)
+        | aexp<qcon> { fbind1 , ... , fbindn }  (labeled update, n >= 1) 
+        | fexp
+```
+
+
+and remove:
+
+```
+
+aexp -> ...
+      | qcon { fbind1 , ... , fbindn }        (labeled construction, n>=0)
+      | aexp<qcon> { fbind1 , ... , fbindn }  (labeled update, n >= 1)
+```
+
+
+and replace
+
+```
+
+pat10 -> ...
+```
+
+
+with:
+
+```
+
+pat10 -> ...
+       | qcon { fpat1 , ... , fpatk }    (labeled pattern, k >= 0)
+```
+
+
+and remove:
+
+```
+
+apat -> ...
+      | qcon { fpat1 , ... , fpatk }    (labeled pattern, k>=0)
+```
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