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# Proposal: [StricterLabelledFieldSyntax](stricter-labelled-field-syntax)
<table><tr><th> Ticket </th>
<th>[\#132](https://gitlab.haskell.org//haskell/prime/issues/132)</th></tr>
<th> [\#132](https://gitlab.haskell.org//haskell/prime/issues/132)
</th></tr>
<tr><th> Dependencies </th>
<th> none
</th></tr>
......@@ -9,8 +11,10 @@
<th> none
</th></tr></table>
## Compiler support
<table><tr><th> GHC </th>
<th> partial (test patch available)
</th></tr>
......@@ -30,36 +34,21 @@
<th> none
</th></tr></table>
## Summary
Make the labelled field syntax stricter, so that unclear code is illegal.
Instead of:
```wiki
data A = A {x :: Int}
y :: Maybe A
y = Just A {x = 5}
```
## Summary
you would need to write:
```wiki
data A = A {x :: Int}
Make the labelled field syntax stricter, so that unclear code is illegal.
y :: Maybe A
y = Just (A {x = 5})
```
## 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
......@@ -72,6 +61,7 @@ main = print $ Just A {x = 5}
(correctly, according to Haskell 98) prints
```wiki
Just A {x = 5}
```
......@@ -79,6 +69,7 @@ Just A {x = 5}
in hugs, but the more easily comprehendible
```wiki
Just (A {x = 5})
```
......@@ -87,8 +78,10 @@ Just (A {x = 5})
in ghci.
According to Haskell 98,
```wiki
A {x = 5}
```
......@@ -97,8 +90,10 @@ 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)
......@@ -110,8 +105,10 @@ 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}
......@@ -120,20 +117,19 @@ 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.**
No additional programs are accepted by this change, and no programs have their behaviour changed. This change only rejects some programs that were previously accepted.
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
......@@ -147,6 +143,7 @@ searchText SearchEntry {entryState = IMode xs ys} = reverse xs ++ ys
and from `Cabal`:
```wiki
configure ... = do
...
......@@ -177,31 +174,21 @@ register pkg@PackageDescription { library = Just lib }
= ...
```
## Result
There was not a consensus to make this change.
## 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)).
This darcs patch is an example of real-world confusion caused by the current parsing:
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)).
```wiki
Sun Feb 21 12:59:33 GMT 2010 Eric Kow <kowey@darcs.net>
* Resolve issue1750: uncover help text for darcs show pristine.
f r { foo = bar } means f (r { foo = bar })
whereas what we really wanted was (f r) { foo = bar }
```
## Report Delta
In [ Section 3](http://haskell.org/onlinereport/exps.html#sect3) replace:
```
exp10 -> ...
......@@ -209,8 +196,12 @@ exp10 -> ...
```
with:
```
exp10 -> ...
......@@ -221,8 +212,12 @@ recexp -> qcon { fbind1 , ... , fbindn } (labeled construction, n >= 0)
```
and remove:
```
aexp -> ...
......@@ -231,48 +226,72 @@ aexp -> ...
```
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 -> ...
......@@ -280,8 +299,12 @@ pat10 -> ...
```
and remove:
```
apat -> ...
......@@ -289,8 +312,12 @@ apat -> ...
```
In [ Section 9.5](http://haskell.org/onlinereport/syntax-iso.html#sect9.5) replace:
```
exp10 -> ...
......@@ -298,8 +325,12 @@ exp10 -> ...
```
with:
```
exp10 -> ...
......@@ -310,8 +341,12 @@ recexp -> qcon { fbind1 , ... , fbindn } (labeled construction, n >= 0)
```
and remove:
```
aexp -> ...
......@@ -320,16 +355,24 @@ aexp -> ...
```
and replace
```
pat10 -> ...
```
with:
```
pat10 -> ...
......@@ -337,10 +380,17 @@ pat10 -> ...
```
and remove:
```
apat -> ...
| qcon { fpat1 , ... , fpatk } (labeled pattern, k>=0)
```
\ No newline at end of file
```