Increase the "width" of ghci

Question

When an output line is too long in GHCI, it is broken:

> :i bar
bar :: Lens' (Foo a0) Int   -- Defined at NewType_makeLenses.hs:7:1
> :i baz
baz :: Lens (Foo a0) (Foo a1) a0 a1
    -- Defined at NewType_makeLenses.hs:7:1

Is there a way to set the maximal length of the lines?

Answer 1

There are three options that control pretty printing:

-dppr-debug         Turn on debug printing (more verbose)
-dppr-user-length   Set the depth for printing expressions in error msgs    
-dppr-cols⟨N⟩       Set the width of debugging output. For example -dppr-cols200

You're looking for -dppr-cols. Its default value is 100. You can set it to any other value, either when you call GHCi or with :set.

Comparison of the the options

Without -dppr-cols

$ ghci NewType_makeLenses.hs
[1 of 1] Compiling Main             ( NewType_makeLenses.hs, interpreted )  
Ok, modules loaded: Main.                                                   
> :i bar                                                                    
bar :: Lens' (Foo a0) Int       -- Defined at NewType_makeLenses.hs:9:1     
> :i baz                                                                    
baz :: Lens (Foo a0) (Foo a1) a0 a1                                         
        -- Defined at NewType_makeLenses.hs:10:1   

With -dppr-cols140

$ ghci -dppr-cols140 NewType_makeLenses.hs
[1 of 1] Compiling Main             ( NewType_makeLenses.hs, interpreted )
Ok, modules loaded: Main.
> :i bar
bar :: Lens' (Foo a0) Int       -- Defined at NewType_makeLenses.hs:9:1
> :i baz
baz :: Lens (Foo a0) (Foo a1) a0 a1     -- Defined at NewType_makeLenses.hs:10:1

With :set -dppr-cols140

$ ghci NewType_makeLenses.hs
[1 of 1] Compiling Main             ( NewType_makeLenses.hs, interpreted )
Ok, modules loaded: Main.
> :set -dppr-cols140
> :i bar
bar :: Lens' (Foo a0) Int       -- Defined at NewType_makeLenses.hs:9:1
> :i baz
baz :: Lens (Foo a0) (Foo a1) a0 a1     -- Defined at NewType_makeLenses.hs:10:1

Bonus: how did I find this?

I didn't look into the flags, instead, I looked into GHC's source code:

$ git clone --depth=1 https://github.com/ghc/ghc.git && cd ghc

Next, I look for a string that starts with "Defined:

$ grep -C2 "\"Defined" -r . --exclude-dir=testsuite
./compiler/basicTypes/Name.hs-ppr_z_occ_name occ = ztext (zEncodeFS (occNameFS occ))
./compiler/basicTypes/Name.hs-
./compiler/basicTypes/Name.hs:-- Prints (if mod information is available) "Defined at <loc>" or
./compiler/basicTypes/Name.hs:--  "Defined in <mod>" information for a Name.
./compiler/basicTypes/Name.hs-pprDefinedAt :: Name -> SDoc
./compiler/basicTypes/Name.hs:pprDefinedAt name = text "Defined" <+> pprNameDefnLoc name
./compiler/basicTypes/Name.hs-
./compiler/basicTypes/Name.hs-pprNameDefnLoc :: Name -> SDoc

SDoc seems interesting.

$ grep "data SDoc" -r . --exclude-dir=testsuite
./compiler/utils/Outputable.hs:data SDocContext = SDC
./compiler/utils/Outputable.hs-boot:data SDoc

Outputable.hs includes printForUser, which uses pprCol dflags together with printDoc from Pretty. printDoc is defined as

printDoc :: Mode -> Int -> Handle -> Doc -> IO ()
-- printDoc adds a newline to the end
printDoc mode cols hdl doc = printDoc_ mode cols hdl (doc $$ text "")

and pprCol is defined in compiler/main/DynFlags.hs, where it corresponds to -dppr-cols. You can just grep your way through GHC :).