Better error for failed MEngine session (#118)

Author: musm

src/engine.jl

@@ -17,7 +17,15 @@ mutable struct MSession
 
     function MSession(bufsize::Integer = default_output_buffer_size)
         ep = ccall(eng_open[], Ptr{Void}, (Ptr{UInt8},), default_startcmd)
-        ep == C_NULL && throw(MEngineError("failed to open a MATLAB engine session"))
+         if ep == C_NULL
+            Base.warn_once("Confirm MATLAB is installed and discoverable.")
+            if iswindows()
+                Base.warn_once("Ensure `matlab -regserver` has been run in a Command Prompt as Administrator.")
+            elseif islinux()
+                Base.warn_once("Ensure `csh` is installed; this may require running `sudo apt-get install csh`.")
+            end
+            throw(MEngineError("failed to open MATLAB engine session"))
+        end
         # hide the MATLAB command window on Windows
         iswindows() && ccall(eng_set_visible[], Cint, (Ptr{Void}, Cint), ep, 0)
 
@@ -54,7 +62,7 @@ end
 function close(session::MSession)
     # close a MATLAB Engine session
     ret = ccall(eng_close[], Cint, (Ptr{Void},), session)
-    ret != 0 && throw(MEngineError("failed to close a MATLAB engine session (err = $ret)"))
+    ret != 0 && throw(MEngineError("failed to close MATLAB engine session (err = $ret)"))
     session.ptr = C_NULL
     return nothing
 end
@@ -137,7 +145,7 @@ eval_string(stmt::String) = eval_string(get_default_msession(), stmt)
 function put_variable(session::MSession, name::Symbol, v::MxArray)
     # put a variable into a MATLAB engine session
     ret = ccall(eng_put_variable[], Cint, (Ptr{Void}, Ptr{UInt8}, Ptr{Void}), session, string(name), v)
-    ret != 0 && throw(MEngineError("failed to put the variable $(name) into a MATLAB session (err = $ret)"))
+    ret != 0 && throw(MEngineError("failed to put variable $(name) into MATLAB session (err = $ret)"))
     return nothing
 end
 
@@ -148,7 +156,7 @@ put_variable(name::Symbol, v) = put_variable(get_default_msession(), name, v)
 
 function get_mvariable(session::MSession, name::Symbol)
     pv = ccall(eng_get_variable[], Ptr{Void}, (Ptr{Void}, Ptr{UInt8}), session, string(name))
-    pv == C_NULL && throw(MEngineError("failed to get the variable $(name) from a MATLAB session"))
+    pv == C_NULL && throw(MEngineError("failed to get variable $(name) from MATLAB session"))
     return MxArray(pv)
 end