REPL: offer to try to install package if not found

stdlib/REPL/src/REPL.jl

@@ -776,9 +776,44 @@ find_hist_file() = get(ENV, "JULIA_HISTORY",
 
 backend(r::AbstractREPL) = r.backendref
 
+# Allows an external package to add hooks into the code loading
+# return true if all packages could be installed, false if not
+# to e.g. install packages on demand
+const install_packages_hooks = Any[]
+
 function eval_with_backend(ast, backend::REPLBackendRef)
+    mods_to_be_loaded = modules_to_be_loaded(ast)
+    mods_not_found = list_unfindable_modules(mods_to_be_loaded)
+    !isempty(mods_not_found) && pass_to_install_package_hooks(mods_not_found)
     put!(backend.repl_channel, (ast, 1))
-    return take!(backend.response_channel) # (val, iserr)
+    resp = take!(backend.response_channel) # (val, iserr)
+    return resp
+end
+
+function modules_to_be_loaded(ast, mods = Symbol[])
+    if ast.head in [:using, :import]
+        for arg in ast.args
+            if first(arg.args) isa Symbol # i.e. `Foo`
+                push!(mods, first(arg.args))
+            else # i.e. `Foo: Bar`
+                push!(mods, first(first(arg.args).args))
+            end
+        end
+    end
+    for arg in ast.args
+        arg isa Expr && modules_to_be_loaded(arg, mods)
+    end
+    return mods
+end
+
+function list_unfindable_modules(mods)
+    filter(mod -> isnothing(Base.identify_package(String(mod))), mods)
+end
+
+function pass_to_install_package_hooks(mods)
+    for f in install_packages_hooks
+        f(mods) && return
+    end
 end
 
 function respond(f, repl, main; pass_empty::Bool = false, suppress_on_semicolon::Bool = true)