Initially, I had written hackage-fetch to see if there was any use of coelgot anywhere on Hackage. At the time, there was not, but this has changed due to my gmpint package. As of writing, it is not surprisingly the only use of co-(Elgot algebra)s on the entirety of Hackage.

And Hackage is big! It's 12626 packages as of writing, totaling 37 million lines of code, 21.5 million of which are Haskell. You can see the table produced by polyglot below:

-------------------------------------------------------------------------------
Language             Files       Lines         Code     Comments       Blanks
-------------------------------------------------------------------------------
Alex                   200       35645        28248         2370         5027
Agda                    30        1542         1166           44          332
Assembly                42       33366        33065            0          301
ATS                     34        4639         3973          305          361
Autoconf               359      112331       109199            0         3132
Automake                13         795          667            0          128
Bash                  1081       47132        40230            0         6902
Batch                   29         680          626            0           54
Brainfuck                5          16           16            0            0
C                     4505     3318644      2953890        28369       336385
C--                      9        2011         1625           15          371
C++                   1950     1990678      1796681        58107       135890
C Header              5805     1981250      1841160            0       140090
Cabal                13890      926906       832160        20838        73908
Cabal Project          112         335          315            8           12
Cassius                 47        1679         1390            0          289
Coq                     11        5384         5102            0          282
CSS                    643      128135       118444            0         9691
Dhall                  429        3060         2972            0           88
Elm                     60        6453         6344           25           84
Emacs Lisp              71       25930        22745            0         3185
Erlang                   1          11            8            0            3
FLTK Data               12        3150         3140            0           10
Fortran                 27        4815         4290            0          525
Go                      29         589          504            3           82
Greencard                4         455          255           96          104
Hamlet                 168        4162         4081            0           81
Happy                  150       72173        55796         4757        11620
Haskell             137811    21552185     16268284      2497001      2786900
HTML                  1235      218891       205478            0        13413
Idris                  621       37050        28754         1170         7126
iPKG                    29         401          288            2          111
Java                   354       50846        44730          466         5650
JavaScript            2164     1255619      1126360         6142       123117
Julius                  25        1139         1042            0           97
Jupyter                  3         756          756            0            0
Lex                    111         380          376            0            4
LLVM                    32        3368         2793            0          575
Lua                     73       16874        13080         1115         2679
M4                      57        5451         4567            0          884
Madlang                 19         401          380            0           21
Makefile               639       27039        21387            0         5652
Markdown              7021      407296       295252            0       112044
Nix                     32       23896        19527            0         4369
Objective C             31       11247         9524            0         1723
OCaml                    1           9            7            0            2
Perl                    29       12537        11693            0          844
PHP                      6        1610         1474            0          136
Plaintext             1873     4499766      4437368            0        62398
PowerShell               6         471          384            0           87
Python                 103       27920        25817            0         2103
PureScript             624        8357         6218            0         2139
R                       20        1340         1197            0          143
Ruby                     9         339          295            0           44
Rust                     1          12            3            5            4
Scala                    2         100           78            0           22
Swift                    7         630          592            0           38
TCL                      5        1109          892            0          217
TeX                   1528       49390        43637            0         5753
TOML                   163        1497         1219            0          278
VHDL                     4         943          821            0          122
Vimscript               84        5118         4703            0          415
YAML                  4252      405772       371414            0        34358
-------------------------------------------------------------------------------
Total               188665    37336109     30813462      2620727      3901920
-------------------------------------------------------------------------------

So there's a lot of different stuff on Hackage. Some of that is unsurprising (Elm, PureScript, Dhall, Agda, and Idris are implemented in Haskell), but some of it is more off-beat (VHDL, Coq, ATS).

C is the second-most popular language, with 3.3 million lines of C on Hackage. This is somewhat surprising - it is one seventh as common as Haskell! Perhaps due to its prolixity, but nonetheless striking.

That's our brief tour of the ecosystem. Hopefully it manages to impress upon you the scale of extant Haskell, as well as some of the cross-pollination happening with other ecosystems.