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Explain what type errors have to do with purity #65

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34 changes: 20 additions & 14 deletions src/HL/View/Home/Features.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -38,20 +38,26 @@ purefunc =
\ mutate any of its arguments.")
haskellPre "square :: Int -> Int\n\
\square x = x * x"
p_ (do "The following string concatenation is okay:")
haskellPre "\"Hello: \" ++ \"World!\" "
p_ (do "The following string concatenation is a type error:")
rejectedHaskellPre "Type error" "\"Name: \" ++ getLine"
p_ (do "Because "
code_ "getLine"
" has type "
code_ "IO String"
" and not "
code_ "String"
", like "
code_ "\"Name: \""
" is. So by the type system you cannot mix and \
\match purity with impurity.")
p_ (do "The following function takes an integer, prints the list of all numbers \
\from zero to that number, and returns its argument. Printing is an "
code_ "IO"
" side-effect, so the function's type must indicate that it describes an "
code_ "IO"
"computation.")
haskellPre "printUpTo :: Int -> IO Int\n\
\printUpTo x = do print [0..x]\n\
\ return x"
p_ (do "Since "
code_ "Int"
" and "
code_ "IO Int"
" are different types, the type system can guarantee that a function of type "
code_ "Int -> Int"
" will not trigger any surprising side-effect.")
haskellPre "printUpToSquare :: Int -> IO Int\n\
\printUpToSquare x = printUpTo (x * x) -- typechecks"
rejectedHaskellPre "Type error" "printUpToSquare :: Int -> Int\n\
\printUpToSquare x = printUpTo (x * x)"

statically :: Html ()
statically =
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